Seems like the default displayed in the text is inconsistent with what the script assumes.
... Signature check FAILED. Do you want to continue anyway? (yes/no) [no] OK. Proceeding with installation anyway. Checking MD5 checksums of files on the ISO image...OK. Done! ...
vs.
... Do you want to continue anyway? (yes/no) [no] no OK. Installation will not be performed. Exiting...