Check in again a file but emptying an attribute

Version: Windchill 12.1

Someone made a check-in but filled in the wrong attribute field.

Now I need to fix it by leaving it blank, but when I try to check in, it says that I can't do that because the field cannot be removed. Any ideas on why this is happening and how to fix it?

Just to add, the field is a string and have no restrictions, and it's an asm file.