extend-type doesn't identify that a protocol is a protocol

I'm trying to implement this tutorial: http://www.lucagrulla.com/posts/server-sent-events-with-ring-and-compojure/

In one part, I need to extend a core.async type with the following protocol: https://github.com/ring-clojure/ring/blob/master/ring-core/src/ring/core/protocols.clj#L19.

So, I've tried to implement something like this:

src/async_test/protocol_ext.clj

But this fails with interface ring.core.protocols.StreamableResponseBody is not a protocol.

Then, I tried to monkey-patch ring: opened a new file with the correct path, and added the following lines on the bottom of the protocol declaration, inside `extend-protocol` call:

src/ring/core/protocols.clj

Then, it worked. What's happening? Is Clojure ignoring that StreamableResponseBody is a protocol if there's already an `extend-protocol` call?