Media Player Won't Play My MKV's Now?

Okay, so for some reason my media player crashes every time I try to play an MKV, but it'll play flv and rmvb. When I try to open a MKV, it just says that Media Player has stopped working for some reason.

I'm working with build 7100. Would upgrading to the RTM fix this?

Oh, and I'm using the K-Lite Codec Pack Mega for the codecs

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