This is only if you use manual partitioning.
Edit// An edition where this is fixed is now on the download mirrors/
There is a workaround for the bug in the installer. Since the bug is not in Ubiquity itself but in the KDE frontend, the solution is to use the Gnome frontend itself:
sudo apt-get install ubiquity-frontend-gtk
sudo apt-get remove ubiquity-frontend-kde
That is - instead of double clicking the installer give the above commands in a terminal
NOTE: You will probably get some error message in the terminal (Konsole in KDE) about some input device - ignore it (them)
Do NOT close Konsole - it must be open as long as the installer runs.
This bug must be fixed by Ubuntu and was unfortunately not discovered in time, it can't be fixed by Mint