diff --git a/src/fsharp/TypeChecker.fs b/src/fsharp/TypeChecker.fs
index c3f4412a20d..e071ac09073 100755
--- a/src/fsharp/TypeChecker.fs
+++ b/src/fsharp/TypeChecker.fs
@@ -10012,11 +10012,13 @@ and TcMethodArg  cenv env  (lambdaPropagationInfo, tpenv) (lambdaPropagationInfo
                     // The loop variable callerLambdaTyOpt becomes None if something failed.
                     let rec loop callerLambdaTy lambdaVarNum = 
                         if lambdaVarNum < numLambdaVars then
-                            let col = [ for row in prefixOfLambdaArgsForEachOverload -> row.[lambdaVarNum] ]
-                            // Check if all the rows give the same argument type
-                            if col |> ListSet.setify (typeEquiv cenv.g) |> isSingleton then 
-                                let calledLambdaArgTy = col.[0]
-                                // Force the caller to be a function type. 
+                            let calledLambdaArgTy = prefixOfLambdaArgsForEachOverload.[0].[lambdaVarNum]
+                            let allRowsGiveSameArgumentType =
+                                prefixOfLambdaArgsForEachOverload
+                                |> Array.forall (fun row -> typeEquiv cenv.g calledLambdaArgTy row.[lambdaVarNum]) 
+
+                            if allRowsGiveSameArgumentType then
+                                // Force the caller to be a function type.
                                 match UnifyFunctionTypeUndoIfFailed cenv env.DisplayEnv mArg callerLambdaTy with 
                                 | Some (callerLambdaDomainTy, callerLambdaRangeTy) ->
                                     if AddCxTypeEqualsTypeUndoIfFailed env.DisplayEnv cenv.css mArg calledLambdaArgTy callerLambdaDomainTy then