> This might be better: "No files specified"
> Or this: "No files chosen"
> [...]
> The return value of `dired-get-marked-files' needs to be tested as
> soon as it's available.  When it is nil we must not call
> `dired-read-shell-command' or do anything else - just raise a
> `user-error'.
>
> This is a general change that needs to be looked for and made
> wherever appropriate.

Agreed.  Simplicity is the hallmark of Emacs.