diff --git a/src/Builder/FrontendDialectTransformer.cpp b/src/Builder/FrontendDialectTransformer.cpp
index fc06081..c03c9f8 100644
--- a/src/Builder/FrontendDialectTransformer.cpp
+++ b/src/Builder/FrontendDialectTransformer.cpp
@@ -485,18 +485,24 @@ private:
     buildOutputAndOperation<mlir::ONNXSliceOp>(node, in, nIn, nOut);
   }
 
+  void ImportCustomNode(const onnx::NodeProto &node) {
+    llvm::StringRef opName = node.op_type();
+
+    mlir::emitWarning(UnknownLoc(), "Could not find op importer: assuming this "
+                                    "represents a custom operator.");
+  }
+
   void ImportNode(const onnx::NodeProto &node) {
     llvm::StringRef opName = node.op_type();
 
-    // the following code is generated by gen_doc.py
-    // refer to Dialect/ONNX/ONNXOps.td for details
-    // when the input or output of then op does not match the specification,
-    // the generic operator is used
-    // one known reeason is the optional input
-
-    auto found = import_handler_map_.find(opName.str());
-    assert(found != import_handler_map_.end() && "Could not find op importer");
-    (this->*(found->second))(node);
+    // look up handler for the opName. If not found, create a node
+    // for a custom op, and issue a warning.
+    auto handler = import_handler_map_.find(opName.str());
+    if (handler != import_handler_map_.end()) {
+      (this->*(handler->second))(node);
+    } else {
+      ImportCustomNode(node);
+    }
   }
 
   void InitHandlerMap() {