diff --git a/lib/ansible/plugins/action/assert.py b/lib/ansible/plugins/action/assert.py
index 4028201130..a3b6af811a 100644
--- a/lib/ansible/plugins/action/assert.py
+++ b/lib/ansible/plugins/action/assert.py
@@ -27,7 +27,7 @@ class ActionModule(ActionBase):
     ''' Fail with custom message '''
 
     TRANSFERS_FILES = False
-    _VALID_ARGS = frozenset(('fail_msg', 'msg', 'that'))
+    _VALID_ARGS = frozenset(('fail_msg', 'msg', 'success_msg', 'that'))
 
     def run(self, tmp=None, task_vars=None):
         if task_vars is None: