diff --git a/src/frontend/define.c b/src/frontend/define.c
index 3dc73d0ae..d9f7d3e50 100644
--- a/src/frontend/define.c
+++ b/src/frontend/define.c
@@ -420,6 +420,8 @@ ntharg(int num, struct pnode *args)
                 else
                     return (NULL);
             }
+            if (!ptry)
+                return NULL;
             ptry = ptry->pn_right;
         }