My Sony Ericsson K800i is not playing wmvs!?

I am sure that the Sony Ericsson K800i can play wmvs- it recognises the file type. But whenever I put a wmv on my phone, it does not play them and an error message comes up saying 'cannot play this video.' The video is not DRM protected, so what's the matter? If it's the resolution, how can I convert it into a suitable one?