Eli Zaretskii <eliz@gnu.org> writes:

> Thanks, but I don't understand why we need the error message.  Isn't
> it enough to pass nil as 2nd argument to shell-command?

The error message is just for noninteractive cases where a caller passes
a numeric argument as the second argument of `async-shell-command'.  For
interactive cases, a nil value will be always passed to `shell-command'.

Example:
    Before patch:
        (async-shell-command "ls" 1) => error: (wrong-type-argument stringp 1)
    After patch:
        (async-shell-command "ls" 1) => error: (error "Invalid output buffer")

Here is a patch without the error message, in case it's preferable: