diff --git a/quic/core/quic_types.cc b/quic/core/quic_types.cc
index 2fe951b..a8b8db9 100644
--- a/quic/core/quic_types.cc
+++ b/quic/core/quic_types.cc
@@ -99,7 +99,8 @@
     RETURN_STRING_LITERAL(VERSION_NEGOTIATION_ERROR);
     RETURN_STRING_LITERAL(PROTOCOL_VIOLATION);
     RETURN_STRING_LITERAL(INVALID_MIGRATION);
-    // No default -- compiler will catch any adds/changes then.
+    default:
+      return QuicStrCat("Unknown Transport Error Code Value: ", c);
   }
 }