diff --git a/src/Builder/FrontendDialectTransformer.cpp b/src/Builder/FrontendDialectTransformer.cpp
index 21c66a0..6396f9c 100644
--- a/src/Builder/FrontendDialectTransformer.cpp
+++ b/src/Builder/FrontendDialectTransformer.cpp
@@ -309,7 +309,13 @@ private:
     int expectedNumOperands = T::getNumberOfOperands();
     int expectedNumResults = T::getNumberOfResults();
     for (const auto &item : node.input())
-      if (initializedTensors.ContainKey(legalize_name(item))) {
+      if (item.empty()) {
+        // Optional inputs using empty string will be imported as NoneType.
+        if (!none_)
+          none_ = builder_.create<mlir::ConstantOp>(
+              UnknownLoc(), builder_.getUnitAttr());
+        inputs.emplace_back(none_);
+      } else if (initializedTensors.ContainKey(legalize_name(item))) {
         inputs.push_back(initializedTensors.EmitInitializerForInputTensor(
             UnknownLoc(), builder_, legalize_name(item)));
       } else if (frontend_symbols_.ContainKey(legalize_name(item))) {