diff --git a/django_jsonform/validators.py b/django_jsonform/validators.py
index 07a95f7..bcf5633 100644
--- a/django_jsonform/validators.py
+++ b/django_jsonform/validators.py
@@ -299,6 +299,7 @@ def validate_string(self, schema, data, coords, raise_exc=False):
             trimmed_data = data.strip()
         else:
             trimmed_data = data
+            data = data or ""
 
         is_empty = not trimmed_data