Help, please: how do I get a non-zero (minibuffer-depth)?

[Alan Mackenzie]

Hi, Emacs!

When the command loop processes an (interactive "@"), it throws an error
when the pertinent mouse event was in an "inactive" minibuffer.

This is when (amongst other things)

    minibuf_level > 0

.  This is equivalent to the lisp

    (> (minibuffer-depth) 0)

.  Would somebody please give me a quick tip, and tell me how to
generate a non-zero minibuffer-depth.  Thanks in advance!

-- 
Alan Mackenzie (Nuremberg, Germany).