Custom self-closing tags don't compile correctly

[iandoesallthethings]

I'm working on an app that will use edge side includes for some content.

My code: esi:include src="user_image" /

Expected result: <esi:include src="user_image" />

Actual result:

<esi:include src="user_image">
  /next tag nested inside for some reason
</esi:include>

I also tried defining a shortcut in slim.rb in case the colon was messing with it somehow, but the result is the same.