do-release-upgrade 14.10 to 15.04 issue

Hi, i had Lubuntu 14.04 installed and recently decided to upgrade it. I followed this tutorial http://askubuntu.com/questions/618741/update-ubuntu-14-04-to-15-04 . While I had no problem upgrading 14.04 -> 14.10 during 14.10 -> 15.04 upgrade I immediately encountered this error:

File "/usr/bin/do-release-upgrade", line 127, in <module>
 "%(url)s\n") % { 'url' : url })
ValueError: unsupported format character '?' (0xa) at index 55

So I changed /usr/bin/do-realease-upgrade 127 line

"%(url)s\n") % { 'url' : url })

with

%s\n" % url )

which solved this problem for me and installation continued. I had Python 3.4.2 installed. The question is quite silly: should it be considered as bug or I just did something wrong?