diff --git a/anki/template/template.py b/anki/template/template.py
index 33bfd6ea8..9431d30cc 100644
--- a/anki/template/template.py
+++ b/anki/template/template.py
@@ -192,7 +192,7 @@ class Template(object):
             else:
                 # hook-based field modifier
                 mod, extra = re.search("^(.*?)(?:\((.*)\))?$", mod).groups()
-                txt = runFilter('fmod_' + mod, txt or '', extra, context,
+                txt = runFilter('fmod_' + mod, txt or '', extra or '', context,
                                 tag, tag_name);
                 if txt is None:
                     return '{unknown field %s}' % tag_name