Profiler.BeginSample doesn't seem to work in CoRoutine?

Unity 5.3.3f, Windows x64, Visual Studio 2015 Community, all targets, running on Windows 7 x64.

Hi. In the process of trying to measure performance of various parts of the code I've added some BeginProfiler("label") calls in some subroutines which I suspect are generating an amount of garbage.

An example is:

 Profiler.BeginSample("frequentUpdate");
 
 while (_timer > 0 && !_exitTest) 
 {
 // does some things I know to be GC heavy including string manipulation
 // does some more stuff
 yield return new WaitForEndOfFrame();
 }
 
 Profiler.EndSample();

And I see nothing relating to this tag in the active profiler window. I have made sure that my profiler frame pointer is on a frame where this is executing, but still nada.

I'm thinking that either,

a) tagged samples don't work in a coroutine, or b) the yield clause to the Enumerator is causing this to break

But the documentation isn't showing me anything helpful, neither is Google.

Anyone got an actual answer?

Thanks