diff --git a/multiselectfield/db/fields.py b/multiselectfield/db/fields.py
index 7d4eef6..a11702b 100644
--- a/multiselectfield/db/fields.py
+++ b/multiselectfield/db/fields.py
@@ -54,7 +54,13 @@ class MultiSelectField(models.CharField):
         self.max_choices = kwargs.pop('max_choices', None)
         super(MultiSelectField, self).__init__(*args, **kwargs)
         self.max_length = get_max_length(self.choices, self.max_length)
-        self.validators[0] = MaxValueMultiFieldValidator(self.max_length)
+
+        validator = MaxValueMultiFieldValidator(self.max_length)
+        if not self.validators:
+            self.validators.append(validator)
+        else:
+            self.validators[0] = validator
+
         if self.min_choices is not None:
             self.validators.append(MinChoicesValidator(self.min_choices))
         if self.max_choices is not None: