Line 21 in run_upgrader.sh checks the file type of the current shell:
if LANG=C file -- "$SHELL" | grep 64-bit >& /dev/null;
However, in my case I'm using zsh, which is sym-linked. Adding -L to follow symbolic links will fix the problem:
if LANG=C file -L -- "$SHELL" | grep 64-bit >& /dev/null;
Cheers,
Stuart