associations – KeySelect via Except[StringMatchQ[Pattern]]failure

I have a Association asc that has mixed type Keys:

asc = <|"b1" -> 2, "a1" -> 1, c3 -> 3 |>;

I try to select in order to exclude a chain pattern through No @ StringMatchQ but without success:

KeySelect[asc, ! StringMatchQ["b" ~~ __?DigitQ]](* <||> *)

The selection based on StringMatchQ by itself it works, but I receive the warning that a Rope it was expected as Key for the third association:

KeySelect[asc, StringMatchQ["b" ~~ __?DigitQ]](*
StringMatchQ: String or list of expected strings ....
<|"b1" -> 2 |>
*)

I would like to understand: