diff --git a/sqlor/filter.py b/sqlor/filter.py
index e41c7e6..1bbd827 100755
--- a/sqlor/filter.py
+++ b/sqlor/filter.py
@@ -127,7 +127,7 @@ class DBFilter(object):
 		assert op in self.operators
 
 		if op in  ['IS NULL', 'IS NOT NULL']:
-			return f'{fj['field']} {op}'
+			return f"{fj['field']} {op}"
 
 		var = fj.get('var')
 		if var and not var in ns.keys():