bug#18046: 24.4.50; wrong-type-argument

[Drew Adams]

(Subject should have mentioned `rectangle--string-preview'.)

Forgot to add that it makes no difference whether the region
is active or the rectangle is non-empty.

This is the culprit code:

(when (equal str "")
      (setq str (or (car-safe minibuffer-default)
                    (if (stringp minibuffer-default)
                        minibuffer-default))))

It sets STR to nil.