Emacs insert empty string

[Kenneth Brun Nielsen]

Hi,

I'm a happy user of Emacs 23.2.1.

One thing annoys me, though.

The string rectangle insert function (CTRL-x rt) is neat. But
sometimes you need to insert an empty string. I cannot do this,
because if I type an empty string (i.e. press Enter), then it insert
the "default" string, which is equal to the last inserted string.

How can I insert an empty string?

Best regards,
Kenneth