Nero does that when it expects an MPG or AVI file for conversion to DVD. What Nero version do you use? In the meantime you can try it the old fashioned way:

1. New Project - DVD Data
2. File format ISO1
3. Somewhere you'll see a checkbox "UDF compatibility". Check it.
4. In your new blank project, generate two folders: One named "VIDEO_TS", the other one named "AUDIO_TS"
5. Move your content into the VIDEO_TS folder, leaving AUDIO_TS empty.
6. Verify all is fine, then burn.

Bon apetit.