diff --git a/mlir/lib/Dialect/Func/Transforms/FuncConversions.cpp b/mlir/lib/Dialect/Func/Transforms/FuncConversions.cpp
--- a/mlir/lib/Dialect/Func/Transforms/FuncConversions.cpp
+++ b/mlir/lib/Dialect/Func/Transforms/FuncConversions.cpp
@@ -29,6 +29,11 @@
                                            convertedResults)))
       return failure();
 
+    // If this isn't a one-to-one type mapping, we don't know how to aggregate
+    // the results.
+    if (callOp->getNumResults() != convertedResults.size())
+      return failure();
+
     // Substitute with the new result types from the corresponding FuncType
     // conversion.
     rewriter.replaceOpWithNewOp<CallOp>(
diff --git a/mlir/test/Transforms/test-legalizer.mlir b/mlir/test/Transforms/test-legalizer.mlir
--- a/mlir/test/Transforms/test-legalizer.mlir
+++ b/mlir/test/Transforms/test-legalizer.mlir
@@ -317,3 +317,16 @@
   %0 = "test.passthrough_fold"(%arg) : (f32) -> (i32)
   "test.return"(%0) : (i32) -> ()
 }
+
+// -----
+
+// expected-remark @below {{applyPartialConversion failed}}
+module {
+  func.func private @callee(%0 : f32) -> f32
+
+  func.func @caller( %arg: f32) {
+    // expected-error @below {{failed to legalize}}
+    %1 = func.call @callee(%arg) : (f32) -> f32
+    return
+  }
+}