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?