Build system tries to find a title line for non-text files

Using this branch: https://code.launchpad.net/~dpm/help-app/test-animated-gif

make web
make -C edit-here web
make[1]: Entering directory '/tmp/help/edit-here'
./generate-translations
Traceback (most recent call last):
  File "./generate-translations", line 17, in <module>
    main()
  File "./generate-translations", line 12, in main
    translations.generate_translations()
  File "/tmp/help/edit-here/translations.py", line 278, in generate_translations
    self.documents.write_translated_markdown(self.po, self.po4a)
  File "/tmp/help/edit-here/translations.py", line 237, in write_translated_markdown
    title_line = output.split('\n')[0].split('Title: ')[1]
IndexError: list index out of range
Makefile:67: recipe for target 'web' failed
make[1]: *** [web] Error 1
make[1]: Leaving directory '/tmp/help/edit-here'
Makefile:20: recipe for target 'web' failed
make: *** [web] Error 2

It seems that the folder added to contain the gif file is scanned on build, and the build system gets confused about not finding a Title: line for the gif file?