diff --git a/python/src/trezorlib/mapping.py b/python/src/trezorlib/mapping.py
index d50324d586..532277078f 100644
--- a/python/src/trezorlib/mapping.py
+++ b/python/src/trezorlib/mapping.py
@@ -62,7 +62,9 @@ class ProtobufMapping:
         """
         wire_type = self.class_to_type_override.get(type(msg), msg.MESSAGE_WIRE_TYPE)
         if wire_type is None:
-            raise ValueError("Cannot encode class without wire type")
+            raise ValueError(
+                f'Cannot encode class "{type(msg).__name__}" without wire type'
+            )
 
         buf = io.BytesIO()
         protobuf.dump_message(buf, msg)