diff --git a/src/django_mysql/models/fields/sets.py b/src/django_mysql/models/fields/sets.py
index f442fe04..06212926 100644
--- a/src/django_mysql/models/fields/sets.py
+++ b/src/django_mysql/models/fields/sets.py
@@ -163,6 +163,7 @@ def check(self, **kwargs: Any) -> list[checks.CheckMessage]:
             and isinstance(self.base_field, CharField)
             and self.size
         ):
+            assert self.base_field.max_length is not None
             max_size = (
                 # The chars used
                 (self.size * (self.base_field.max_length))