1 files changed, 2 insertions, 6 deletions
diff --git a/oauthlib/oauth2/rfc6749/utils.py b/oauthlib/oauth2/rfc6749/utils.py
index 6a8e24b..49c3a2f 100644
--- a/oauthlib/oauth2/rfc6749/utils.py
+++ b/oauthlib/oauth2/rfc6749/utils.py
@@ -24,20 +24,16 @@ def list_to_scope(scope):
     """Convert a list of scopes to a space separated string."""
     if isinstance(scope, unicode_type) or scope is None:
         return scope
-    elif isinstance(scope, (tuple, list)):
+    elif isinstance(scope, (set, tuple, list)):
         return " ".join([unicode_type(s) for s in scope])
-    elif isinstance(scope, set):
-        return list_to_scope(list(scope))
     else:
         raise ValueError("Invalid scope, must be string or list.")
 
 
 def scope_to_list(scope):
     """Convert a space separated string to a list of scopes."""
-    if isinstance(scope, list):
+    if isinstance(scope, (tuple, list, set)):
         return [unicode_type(s) for s in scope]
-    if isinstance(scope, set):
-        scope_to_list(list(scope))
     elif scope is None:
         return None
     else: