diff --git a/sqlparse/engine/grouping.py b/sqlparse/engine/grouping.py
index 86d8fc64..57d257e2 100644
--- a/sqlparse/engine/grouping.py
+++ b/sqlparse/engine/grouping.py
@@ -184,7 +184,7 @@ def match(token):
         return token.match(T.Assignment, ':=')
 
     def valid(token):
-        return token is not None and token.ttype not in (T.Keyword)
+        return token is not None and token.ttype not in (T.Keyword,)
 
     def post(tlist, pidx, tidx, nidx):
         m_semicolon = T.Punctuation, ';'