`Only letters, numbers, underscores and hypens allowed` error on upper case letters

[D4nte]

• Actual: When clicking "use a throwaway account", the error Only letters, numbers, underscores and hypens allowed shows when using uppercase letters.

• Expected: Upper case letters are allowed OR the error specify that only lower case letters are accepted.

• version: b82bf0c0e5d884566f11e92209f573a94641b5cc