diff --git a/caravel/models.py b/caravel/models.py
index f8f042d19c18f..fa16e7ed73ee9 100644
--- a/caravel/models.py
+++ b/caravel/models.py
@@ -428,10 +428,8 @@ def grains(self):
             if self.sqlalchemy_uri.startswith(db_type):
                 return grains
 
-    def dttm_converter(self, dttm, tf=None):
+    def dttm_converter(self, dttm, tf='%Y-%m-%d %H:%M:%S.%f'):
         """Returns a string that the database flavor understands as a date"""
-        if tf is None or tf == '':
-            tf = '%Y-%m-%d %H:%M:%S.%f'
         default = "'{}'".format(dttm.strftime(tf))
         iso = dttm.isoformat()
         d = {