diff --git a/lib/cmdlib.py b/lib/cmdlib.py
index de723653f44f8fcdf140a2e55a5eb617a2311230..0603f95473fd5c7bc8514361edd272cd693971ed 100644
--- a/lib/cmdlib.py
+++ b/lib/cmdlib.py
@@ -10270,7 +10270,8 @@ class _GroupQuery(_QueryBase):
           missing.append(name)
 
       if missing:
-        raise errors.OpPrereqError("Some groups do not exist: %s" % missing,
+        raise errors.OpPrereqError("Some groups do not exist: %s" %
+                                   utils.CommaJoin(missing),
                                    errors.ECODE_NOENT)
 
   def DeclareLocks(self, lu, level):