I found our little mystery frog to have a familiar look to it. I am pretty sure I have nailed it down to genus and that genus is Ptychadena. Looking at a field guide for East Africa, one canidate is Ptychadena mascareniensis (Duméril & Bibron, 1841), Mascarene ridge frog. The photo in the book did closely match yours. Most of the pics I came across on-line for this species showed a stripe down the middle of the dorsum. Yours lacks this feature. However, I did find a picture of one without a stripe. http://calphotos.berkeley.edu/cgi/im...0000+0907+0885
If its not mascareniensis, thats ok. I am 99.9% sure its of the genus Ptychadena, family Ranidae.