Can Kaon Decay Violate Lepton Number Conservation?

[zardiac]

Homework Statement

I am asked to draw a feynman diagram of K^- \rightarrow \mu \nu

Homework Equations

K^- consist of s\bar{u}

The Attempt at a Solution

I wonder what the direcion of the arrows should be, my guess is something like the attached image.
edit: Ah, where the left u shoudl be an s and the right u should be an antiup. (attached the wrong pic)

 

[vela]

That decay can't happen, can it? Doesn't it violate lepton number conservation?