fix test

Author: austinc

pandas/tseries/frequencies.py

@@ -927,7 +927,7 @@ def _get_wom_rule(self):
 
         week_of_months = unique((self.index.day - 1) // 7)
         week_of_months = week_of_months[week_of_months < 4]
-        if len(week_of_months) > 1:
+        if len(week_of_months) == 0 or len(week_of_months) > 1:
             return None
 
         # get which week