Perforce buildQueue REST trigger doesn't respect branchName?

I'm having trouble creating a REST API request that builds a specific branch in TeamCity using the Perforce feature branches support. My request looks like this:

<build><buildType id="ID" branchName="conan"/><comment><text>Test REST API trigger</text></comment></build>

But the response I get back is (200 OK):

<build id="13169" buildTypeId="ID" state="queued" branchName="<default>" defaultBranch="true" href="/httpAuth/app/rest/buildQueue/id:13169"webUrl="http://teamcity/viewQueued.html?itemId=13169">

TC then runs a build for the default branch instead of the specified branch. It appears that it's totally ignoring my branchName specification. Is this unimplemented/broken or am I doing something wrong?