diff --git a/app/models/user.rb b/app/models/user.rb
index ec6acbe3a..f2a213d1c 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -311,7 +311,7 @@ class User
   end
 
   def downcase_username
-    username.downcase!
+    username.downcase! if username
   end