Thread:Extension talk:User Merge and Delete/"Invalid new username" highlighted in red when merging

Installed extension as instructed without difficulties.

"Merge and delete users" shows up on Special Pages.

If I try to merge and delete a user as WikiSysop though I always end up with red error message "Invalid new user name"

Any solutions ? Thank you

Please disregard above question: If I had a proper understanding of merging I had known that the new user must exist before I start "merging"