diff --git a/lib/py/images/pb2dict.py b/lib/py/images/pb2dict.py
index c4ce736e8..6b4a772c7 100644
--- a/lib/py/images/pb2dict.py
+++ b/lib/py/images/pb2dict.py
@@ -224,7 +224,7 @@ def get_bytes_dec(field):
 def is_string(value):
 	# Python 3 compatibility
 	if "basestring" in __builtins__:
-		string_types = basestring
+		string_types = basestring  # noqa: F821
 	else:
 		string_types = (str, bytes)
 	return isinstance(value, string_types)