In Somali, what rule accounts for singular /sun/ changing to plural /sumo/?

I'm working on a Phonological Analysis assignment and I am trying to figure out a rule to formulate that would account for the singular form of 'poison' in Somali changing from /sun/ to plural /sumo/. I have figured out that /sun/ is the underlying form and the plural suffix is /-o/ so I'm just trying to figure out why n changes to m morpheme-finally. This also occurs in 'branch' where singular /la:n/ changes to plural /la:mo/ and so on. Any help is greatly appreciated! Thanks!