diff --git a/src/c3nav/site/views.py b/src/c3nav/site/views.py
index cce05fcc..c2d7e8db 100644
--- a/src/c3nav/site/views.py
+++ b/src/c3nav/site/views.py
@@ -162,6 +162,7 @@ def register_view(request):
     else:
         form = UserCreationForm()
 
+    form.fields['username'].max_length = 20
     for field in form.fields.values():
         field.help_text = None