I don't think it is an error. In UK English, the Oh sound is often used for 0. Saying zero sounds a bit formal and scientific. US English tends to use zero in everyday speech. You'd have to find an alternative TTS voice, unless anyone knows a techhy fix...I don't though.
Here's what I did and it works fine. I downloaded my modified TTS_LOQ folder and extracted it to Primo in addition to the tts folder already there. For iGO 8.3 uninstall the TTS_LOQ folder already there and replace it with mine. Then I put the iGO8 modified louder voices from content\voice in addition [not overwriting] to those voices already in the Primo folder. Now I have a number of TTS English voices. Remember that you also need the .dlls.
After you install my TTS_LOQ folder you'll be able to edit the .lex file. Try this entry: "0"="zero" at the beginning of the file.The "0" is the number zero. Don't forget to save the file before exiting.