Hello,

This is sort of a garbage-in, garbage-out situation, so I'll just let
the reproducer speak for itself:

$ emacs -Q
M-! ./nosuchcommand C-b TAB

- In Emacs 27.1, this simply says "[No match]".
- In Emacs 28.0.60, this errors out:

> completion--some: Args out of range: "./nosuchcomma", 0, 14

I'm attaching the backtrace obtained with M-x toggle-debug-on-error.

I'm not on the most ideal setup for compiling ATM, so I apologize for
not bisecting this (and for not trying with 27.2; Debian stable only has
27.1); I'll get to it eventually if no-one beats me.

AFAICT it's not a very impactful issue; however, since Emacs now throws
an error where it didn't use to, I figure there's a somewhat subtle bug
here with consequences I can't quite evaluate.

Thanks for your time.