diff --git a/google/oauth2/credentials.py b/google/oauth2/credentials.py
index 4cc502fea..47ff65b93 100644
--- a/google/oauth2/credentials.py
+++ b/google/oauth2/credentials.py
@@ -172,6 +172,8 @@ def refresh_token(self):
     @property
     def scopes(self):
         """Optional[str]: The OAuth 2.0 permission scopes."""
+        if isinstance(self._scopes, set):
+            return list(self._scopes)
         return self._scopes
 
     @property