How to 'UnJustify!' text in GNU nano

I enjoy using nano as a respite from my usual GTK-based text editor. I like the simplicity of the interface, and using CTRL-K is the fastest way I know of to edit down long textfiles. However, I have one major gripe: whenever I justify text using CTRL-J, the editor prints the smug little message Can now UnJustify! -- yet I have not been able to find a way to unjustify text. Pressing M-U (which a Google search could reveal, M-U not being mentioned at all in the program's help files) simply seems to cause a glitch. The keyboard becomes unresponsive. Am I missing something?