diff --git a/src/django_fields/fields.py b/src/django_fields/fields.py
index c7c97b9..834c580 100644
--- a/src/django_fields/fields.py
+++ b/src/django_fields/fields.py
@@ -58,7 +58,6 @@ def _get_padding(self, value):
         mod = (len(value) + 2) % self.cipher.block_size
         return self.cipher.block_size - mod + 2
 
-
     def to_python(self, value):
         if self._is_encrypted(value):
             return force_unicode(
@@ -69,6 +68,9 @@ def to_python(self, value):
         return value
 
     def get_db_prep_value(self, value, connection=None, prepared=False):
+        if value is None:
+            return None
+
         value = smart_str(value)
 
         if value is not None and not self._is_encrypted(value):
@@ -192,15 +194,17 @@ def get_internal_type(self):
 
     def to_python(self, value):
         # value is either an int or a string of an integer
-        if isinstance(value, self.number_type):
+        if isinstance(value, self.number_type) or value is None:
             number = value
         else:
             number_text = super(BaseEncryptedNumberField, self).to_python(value)
             number = self.number_type(number_text)
         return number
 
-    # def get_prep_value(self, value):
     def get_db_prep_value(self, value, connection=None, prepared=False):
+        if value is None:
+            return None
+
         number_text = self.format_string % value
         return super(BaseEncryptedNumberField, self).get_db_prep_value(
             number_text,