diff --git a/mlir/lib/Transforms/Utils/DialectConversion.cpp b/mlir/lib/Transforms/Utils/DialectConversion.cpp
--- a/mlir/lib/Transforms/Utils/DialectConversion.cpp
+++ b/mlir/lib/Transforms/Utils/DialectConversion.cpp
@@ -1068,7 +1068,7 @@
     // Handle the case where the conversion was 1->1 and the new operand type
     // isn't legal.
     Type newOperandType = newOperand.getType();
-    if (converter && desiredType && newOperandType != desiredType) {
+    if (converter && desiredType && !converter->isLegal(newOperandType)) {
       // Attempt to materialize a conversion for this new value.
       newOperand = converter->materializeTargetConversion(
           rewriter, loc, desiredType, newOperand);