1 files changed, 1 insertions, 1 deletions

diff --git a/parse/col_map.py b/parse/col_map.py
index ccce865..f1f9e52 100644
--- a/parse/col_map.py
+++ b/parse/col_map.py
@@ -45,7 +45,7 @@ class ColMap(object):
 
         for col in self.col_list:
             if col not in kv:
-                key += (None,)
+                key += (str(None),)
             else:
                 key += (str(kv[col]),)