POSNER, Circuit Judge.
The district court, rejecting a recommendation by the magistrate, granted summary judgment for the defendants in this breach of contract suit, and the plaintiff appeals. The basis of federal jurisdiction is diversity of citizenship. 28 U.S.C. § 1382. The plaintiff, the City of Clinton, Illinois, is a municipal corporation of Illinois, and for diversity purposes a municipal corporation is treated just like a regular business corporation. Moor v. County of Alameda, 411 U.S. 693, 717-18, 93 S.Ct. 1785, 1799-1800, 36 L.Ed.2d 596 (1973); Illinois v. City of Milwaukee, 406 U.S. 91, 97, 92 S.Ct. 1385, 1389, 31 L.Ed.2d 712 (1972). The principal defendant, Moffitt (a real estate developer), is a citizen of Missouri. A Missouri general partnership in which he is a partner is also a defendant, but the citizenship of a partnership is irrelevant to diversity. Great Southern Fire Proof Hotel Co. v. Jones, 177 U.S. 449, 456, 20 S.Ct. 690, 693, 44 L.Ed. 842 (1900); Elston Investment, Ltd. v. David Altman Leasing Corp., 731 F.2d 436 (7th Cir.1984); 13B Wright, Miller & Cooper, Federal Practice and Procedure § 3630 (2d ed. 1984). But since all the other partners are citizens of Missouri as well as Moffitt, and the remaining defendant is not a citizen of Illinois either, the requirement of complete diversity is satisfied. Strawbridge v. Curtiss, 7 U.S. (3 Cranch) 267, 2 L.Ed. 435 (1806); Fidelity & Deposit Co. v. City of Sheboygan Falls, 713 F.2d 1261, 1264 (7th Cir.1983). So there is federal jurisdiction.
The parties agree that Illinois contract law governs the substantive issues. Both the parties and the district judge say that Illinois contract law governs because this is a diversity suit, but that is not accurate. Because it is a diversity suit, Illinois law governs, but it is the whole Illinois law that governs, including Illinois’ conflict of law rules. Casio, Inc. v. S.M. & R. Co., 755 F.2d 528, 530-31 (7th Cir.1985). If those rules pointed to the contract law of another state, that other state’s law would govern. But the parties agree that Illinois contract law governs, and that is all that is necessary to make it govern. Litigants can, by stipulation, formal or informal, agree on the substantive law to be applied to their case, within broad limits not exceeded here. Id. at 531.
With these preliminaries out of the way we turn to the merits. The district judge thought it clear from the terms of the parties’ contract (which was in writing) that Moffitt did not make the promise that the City of Clinton says he broke. If the judge was right in this, then he was right to grant summary judgment for Moffitt. “If a contract is in writing, is unambiguous and contains no uncertain terms, interpretation of the contract is a question of law for the court.” Nerone v. Boehler, 34 Ill.App.3d 888, 890-91, 340 N.E.2d 534, 536 (1976). See also National Tea Co. v. Commerce & Industry Ins. Co., 119 Ill.App.3d 195, 200-01, 74 Ill.Dec. 704, 708-09, 456 N.E.2d 206, 210-11 (1983). The question whether a contract is ambiguous is itself a question of law, Wilson v. Illinois Benedictine College, 112 Ill.App.3d 932, 937, 68 Ill.Dec. 257, 263, 445 N.E.2d 901, 907 (1983), and we give substantial weight to interpretations of state law by a federal district judge sitting in the state whose law is in question, Goldstick v. ICM Realty, 788 F.2d 456, 466 (7th Cir.1986). This precept is especially apropos in a case such as this where the district judge is a former judge of that state’s courts. But we do not merely rubber-stamp the district judge’s determination of state law, and where (as in this case) we find ourselves in strong disagreement with the determination, we must reverse.
Moffitt wanted to build a shopping center on land he owned in the City of Clinton, [343]*343Illinois. To improve access to the shopping center he made a contract with the city by which they agreed to split the cost of building a new street that would run north and south along the western edge of the shopping center. The street would connect Route 54, which runs along the southern edge of the shopping center, and Route 10, which runs roughly parallel to Route 54 north of the shopping center. The shopping center ends about half way between Route 54 and Route 10, and the deal was that Moffitt would build the part of the new street that ran north along his property line and the city would build the rest, linking up with Route 10 and thus completing the connection between the two highways.
The city let contracts for building the street, including the intersections with Route 10 at the north end of the street and Route 54 at the south end. The total cost with the intersections came to $330,000 (we round off all dollar figures to the nearest $1,000). The city billed Moffitt $204,000, covering all the work south of the shopping center’s northern border. Moffitt paid only $99,000, contending that the rest of the $204,000 was for the intersection with Route 54 and that his contract with the city had placed that cost on the city.
The contract says that Moffitt will “pay for the cost of constructing the street north from U.S. Route 54 approximately 520 feet to” the point where the shopping center ends. Moffitt interprets this to mean north from the Route 54 right of way, the northern edge of which was 38 feet beyond the northern edge of the highway itself before the improvements were made. From the right of way to the end of the shopping center is 526 feet; this Moffitt contends is the “approximately 520 feet” to which the contract refers. All the expensive work on the intersection was south of the northern edge of the right of way. Under the city’s theory Moffitt must pay the cost of construction from the northern edge of Route 54 itself to the north end of the shopping center, a distance of 564 feet (526 + 38), which he contends is too long to be “approximately 520 feet.”
He could be right. Intersection work is a lot less straightforward than straight street work, so that if he agreed to pick up the tab for the intersection of the street with Route 54 as well as the street itself he was buying a pig in the poke. But against this it can be pointed out that before the contract was signed an engineer had given Moffitt an estimate of Moffitt’s share of the cost of the street work. The estimate was for $130,000, which included intersection work, as clearly shown on the engineering drawings submitted with the estimate. Now $130,000 is a lot more than $99,000, which is all Moffitt says he owes; and this discrepancy is some evidence for the city’s interpretation of the contract.
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POSNER, Circuit Judge.
The district court, rejecting a recommendation by the magistrate, granted summary judgment for the defendants in this breach of contract suit, and the plaintiff appeals. The basis of federal jurisdiction is diversity of citizenship. 28 U.S.C. § 1382. The plaintiff, the City of Clinton, Illinois, is a municipal corporation of Illinois, and for diversity purposes a municipal corporation is treated just like a regular business corporation. Moor v. County of Alameda, 411 U.S. 693, 717-18, 93 S.Ct. 1785, 1799-1800, 36 L.Ed.2d 596 (1973); Illinois v. City of Milwaukee, 406 U.S. 91, 97, 92 S.Ct. 1385, 1389, 31 L.Ed.2d 712 (1972). The principal defendant, Moffitt (a real estate developer), is a citizen of Missouri. A Missouri general partnership in which he is a partner is also a defendant, but the citizenship of a partnership is irrelevant to diversity. Great Southern Fire Proof Hotel Co. v. Jones, 177 U.S. 449, 456, 20 S.Ct. 690, 693, 44 L.Ed. 842 (1900); Elston Investment, Ltd. v. David Altman Leasing Corp., 731 F.2d 436 (7th Cir.1984); 13B Wright, Miller & Cooper, Federal Practice and Procedure § 3630 (2d ed. 1984). But since all the other partners are citizens of Missouri as well as Moffitt, and the remaining defendant is not a citizen of Illinois either, the requirement of complete diversity is satisfied. Strawbridge v. Curtiss, 7 U.S. (3 Cranch) 267, 2 L.Ed. 435 (1806); Fidelity & Deposit Co. v. City of Sheboygan Falls, 713 F.2d 1261, 1264 (7th Cir.1983). So there is federal jurisdiction.
The parties agree that Illinois contract law governs the substantive issues. Both the parties and the district judge say that Illinois contract law governs because this is a diversity suit, but that is not accurate. Because it is a diversity suit, Illinois law governs, but it is the whole Illinois law that governs, including Illinois’ conflict of law rules. Casio, Inc. v. S.M. & R. Co., 755 F.2d 528, 530-31 (7th Cir.1985). If those rules pointed to the contract law of another state, that other state’s law would govern. But the parties agree that Illinois contract law governs, and that is all that is necessary to make it govern. Litigants can, by stipulation, formal or informal, agree on the substantive law to be applied to their case, within broad limits not exceeded here. Id. at 531.
With these preliminaries out of the way we turn to the merits. The district judge thought it clear from the terms of the parties’ contract (which was in writing) that Moffitt did not make the promise that the City of Clinton says he broke. If the judge was right in this, then he was right to grant summary judgment for Moffitt. “If a contract is in writing, is unambiguous and contains no uncertain terms, interpretation of the contract is a question of law for the court.” Nerone v. Boehler, 34 Ill.App.3d 888, 890-91, 340 N.E.2d 534, 536 (1976). See also National Tea Co. v. Commerce & Industry Ins. Co., 119 Ill.App.3d 195, 200-01, 74 Ill.Dec. 704, 708-09, 456 N.E.2d 206, 210-11 (1983). The question whether a contract is ambiguous is itself a question of law, Wilson v. Illinois Benedictine College, 112 Ill.App.3d 932, 937, 68 Ill.Dec. 257, 263, 445 N.E.2d 901, 907 (1983), and we give substantial weight to interpretations of state law by a federal district judge sitting in the state whose law is in question, Goldstick v. ICM Realty, 788 F.2d 456, 466 (7th Cir.1986). This precept is especially apropos in a case such as this where the district judge is a former judge of that state’s courts. But we do not merely rubber-stamp the district judge’s determination of state law, and where (as in this case) we find ourselves in strong disagreement with the determination, we must reverse.
Moffitt wanted to build a shopping center on land he owned in the City of Clinton, [343]*343Illinois. To improve access to the shopping center he made a contract with the city by which they agreed to split the cost of building a new street that would run north and south along the western edge of the shopping center. The street would connect Route 54, which runs along the southern edge of the shopping center, and Route 10, which runs roughly parallel to Route 54 north of the shopping center. The shopping center ends about half way between Route 54 and Route 10, and the deal was that Moffitt would build the part of the new street that ran north along his property line and the city would build the rest, linking up with Route 10 and thus completing the connection between the two highways.
The city let contracts for building the street, including the intersections with Route 10 at the north end of the street and Route 54 at the south end. The total cost with the intersections came to $330,000 (we round off all dollar figures to the nearest $1,000). The city billed Moffitt $204,000, covering all the work south of the shopping center’s northern border. Moffitt paid only $99,000, contending that the rest of the $204,000 was for the intersection with Route 54 and that his contract with the city had placed that cost on the city.
The contract says that Moffitt will “pay for the cost of constructing the street north from U.S. Route 54 approximately 520 feet to” the point where the shopping center ends. Moffitt interprets this to mean north from the Route 54 right of way, the northern edge of which was 38 feet beyond the northern edge of the highway itself before the improvements were made. From the right of way to the end of the shopping center is 526 feet; this Moffitt contends is the “approximately 520 feet” to which the contract refers. All the expensive work on the intersection was south of the northern edge of the right of way. Under the city’s theory Moffitt must pay the cost of construction from the northern edge of Route 54 itself to the north end of the shopping center, a distance of 564 feet (526 + 38), which he contends is too long to be “approximately 520 feet.”
He could be right. Intersection work is a lot less straightforward than straight street work, so that if he agreed to pick up the tab for the intersection of the street with Route 54 as well as the street itself he was buying a pig in the poke. But against this it can be pointed out that before the contract was signed an engineer had given Moffitt an estimate of Moffitt’s share of the cost of the street work. The estimate was for $130,000, which included intersection work, as clearly shown on the engineering drawings submitted with the estimate. Now $130,000 is a lot more than $99,000, which is all Moffitt says he owes; and this discrepancy is some evidence for the city’s interpretation of the contract. It is also, of course, a lot less than the $204,-000 that the city billed him for, but the city says that the cost of constructing the intersection with Route 54 turned out to be unexpectedly great.
But we are straying beyond the written contract, which we are not allowed to do unless it is ambiguous. Moffitt makes two arguments that it is unambiguous. First he points out that the contract requires him to pay for the cost of constructing “the street,” and he contends that “street” doesn’t include “intersection,” which he regards as an improvement to Route 54. Second, if interpreted to require him to pay for the intersection, the contract would extend the distance for which he must pay from 526 feet to 564 feet, and the second figure cannot be “approximately 520 feet.” The second argument does not impress us. The words “approximately 520 feet” could well be intended as descriptive language rather than language defining Moffitt’s obligations. Moreover, “approximately” has no fixed meaning, and a deviation of less than 10 percent is not necessarily precluded by the use of so vague a word.
The first argument has some force. Suppose the city, in making the intersection, had built south of the northern edge of Route 54 — and apparently it did, by widening the highway southward and adding a third lane in the middle of the highway to [344]*344enable eastbound traffic to turn north onto the new street. A powerful argument could be made against the city’s being allowed to charge Moffitt for the cost of these improvements. His contractual obligation is limited to the street running north from the highway; the third lane is in the highway. But the city told us at argument that it isn’t trying to collect for any part of the cost incurred south of the northern edge of the highway. It just wants Moffitt to pay for the pavement of the new street from the northern edge of Route 54 rather than from 38 feet farther north, and for the widening of Route 54 northward to make it easier for drivers to turn from the highway into the new street. Certainly the pavement of the new street from the point where it joins Route 54 is easily comprehended within the meaning of “street,” and, once this is conceded, it becomes possible to regard the other work north of the previous north edge of Route 54 as a part of Moffitt’s undertaking too. The cost of constructing a street might well be thought to include the cost of constructing the equivalent of an access ramp leading to it. A river includes its delta; why cannot a street include a flared portion at the end, designed to make turning in and out of it easier? By this logic, maybe even the cost of the third lane, though constructed south of the northern edge of Route 54, could be viewed as a part of the cost of constructing the new street. But the city is not pressing the point, so neither shall we.
Even if the word “street” excluded the intersection at one end of it, it would not follow that the street did not begin till the northern edge of the highway right of way, 38 feet north of the highway itself. “[Sjtreet north from TJ.S. Route 54” doesn’t sound to the casual ear like “street north from the right of way that surrounds Route 54.” Moffitt's interpretation of the contract requires inserting “right of way of” before “U.S. Route 54.” A contract that has to be rewritten in this counterintuitive fashion in order to yield the result reached by the district judge cannot be considered unambiguous. It has the odd consequence that the city pays for the first 38 feet of the new street, Moffitt for the next 526, and the city for the remainder. This is possible, but is hardly compelled by the language of the contract.
Under Illinois law, if the contract is ambiguous, its interpretation is still an issue of law rather than one of fact if any extrinsic evidence (evidence beyond the words of the written contract itself) bearing on that interpretation is undisputed. Nerone v. Boekler, supra, 34 Ill.App.3d at 891, 340 N.E.2d at 537; Ridenhour v. Mollman Publishing Co., 66 Ill.App.3d 1049, 1051, 23 Ill.Dec. 36, 37-38, 383 N.E.2d 803, 804-05 (1978). Whether the extrinsic evidence in this case is undisputed, and if so (or not) how the contract should be interpreted, are questions for the district court to decide in the first instance; the court erroneously excluded all the extrinsic evidence from consideration, believing the contract to be clear. The evidence that the district judge refused to consider suggests that the parties, noticing that the shopping center ended roughly halfway up the proposed new street, decided that Moffitt would in effect be responsible for the lower (southern) half, including the intersection with Route 54, and the city for the upper half, including the intersection with Route 10. Such an interpretation, under which the city would win, may be right or wrong, but it is not ruled out by unambiguous contractual language.
Only if it turns out that the extrinsic evidence is disputed need there be a trial; but summary judgment should not have been granted on the basis of the contractual language alone. We point out in this connection that the magistrate and district judge differed on whether the contract was ambiguous, that the judge first adopted the magistrate’s recommendation and then changed his mind, and that he did so (it appears) without the benefit of oral argument on the motion for summary judgment. While none of these factors proves that the contract really is ambiguous, they reinforce our doubts that the parties’ dispute can be resolved summarily, at least on the basis of the contract alone. We add that we found argument in this case extremely helpful in focusing the issues, and [345]*345it is possible that the district judge would have come to the same conclusion as we if he had held argument before ruling on the motion for summary judgment.
Reversed and Remanded.