diff --git a/python/nav/web/crispyforms.py b/python/nav/web/crispyforms.py
index e76985d8be..4b21a7da95 100644
--- a/python/nav/web/crispyforms.py
+++ b/python/nav/web/crispyforms.py
@@ -204,7 +204,7 @@ class FormDiv:
     :param css_classes: Additional CSS classes to apply to the div.
     """
 
-    def __init__(self, fields: Optional[list] = None, css_classes: str = ''):
+    def __init__(self, fields: Optional[list] = None, css_classes: Optional[str] = None):
         self.fields = fields
         self.css_classes = css_classes
         self.template = 'custom_crispy_templates/form_div.html'