Unrecognized character reference \"Ü\"

[tneumueller]

Versions

html-validator: 5.1.17
Node: 12.16.1

Problem

When validating the string "Ü" with the following options, an error is returned:

await validator({
  validator: 'WHATWG',
  "Ü",
  isFragment: true
})

Error: Unrecognized character reference \"Ü\"

Expected behaviour

No error should be returned, because according to the list of named character references of the HTML spec, Ü is a valid character sequence.

The same is true for both Ä and Ö, but not for their lower case counterparts.