Turn off auto-summary, that'll auto-magically summarize at classfull boundaries. Lemme guess, the route is in the range of 128.0.0.0 - 191.0.0.0?
If that doesn't do it then maybe you could describe the route
what version of IOS? IP classless command in your config? Where are you getting this route from - IGP, iBGP, eBGP?