> emacs -Q
>  
> (defcustom foo '(current-time-string)
>   "@@@@@@@@@@@@@@@"
>   :type 'sexp)
>  
> M-x customize-option foo
>  
> Type this into the editable field:
>  
> (current-
>  
> Then hit `M-TAB'.  You get this inappropriate error message:
>  
> "Not in an editable field"
>  
> If completion is not available, the message should say that.  It should
> not claim that the field is not editable when it is.

I attach a patch to fix this.

Best regards,
Mauro.