This paper argues that Nubi, an Arabic creole spoken in Bombo, Uganda (among other places) escapes a categorization as either a tone language or a stress-accent languages. It could be classified as a pitch accent language in the sense of Beckman (1986), but unlike languages like Tokyo Japanese lacks a class of unaccented words.

Beckman, M. E. (1986). Stress and Non-stress Accent. Dordrecht: Foris.

@inproceedings{gussenhoven06_tal,
  title     = {Nubi is not a tone language},
  author    = {Carlos Gussenhoven},
  year      = {2006},
  booktitle = {2nd International Symposium on Tonal Aspects of Languages (TAL 2006)},
  pages     = {57--60},
}