diff --git a/mlir/lib/Parser/TypeParser.cpp b/mlir/lib/Parser/TypeParser.cpp
--- a/mlir/lib/Parser/TypeParser.cpp
+++ b/mlir/lib/Parser/TypeParser.cpp
@@ -312,6 +312,12 @@
   switch (getToken().getKind()) {
   default:
     return (emitError("expected non-function type"), nullptr);
+  case Token::bare_identifier:
+    return (emitError("expected non-function type,"
+                      " but found start of custom operation."
+                      " Did you mean to prefix the identifier with '!'"
+                      " to refer to a dialect type?"),
+            nullptr);
   case Token::kw_memref:
     return parseMemRefType();
   case Token::kw_tensor: