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
@@ -1147,6 +1147,8 @@
                        block, converter, *conversion, mapping, argReplacements)
                  : argConverter.convertSignature(block, converter, mapping,
                                                  argReplacements);
+  if (failed(result))
+    return failure();
   if (Block *newBlock = result.getValue()) {
     if (newBlock != block)
       blockActions.push_back(BlockAction::getTypeConversion(newBlock));