When a user loads Special:GlobalContributions for a username target, they are informed that the tool does not accept usernames. This is because the tool currently only supports IP addresses and ranges, though will be later updated to support temporary accounts (and named accounts too).

However, when entering a username target that does not exist, the form shows the validation error but also has a warning that says that the username does not exist. Given that a username is not a valid target, we should probably be hiding this warning from the page as it may be confusing (as it could imply that existing username targets are valid).

An example of this problem is shown below: