Can't debug the Razor helper - it seems IIS is not compiling with debugging symbols?

I am trying to debug a Razor helper function that runs locally in IIS.

Even when my helper code reads:

System.Diagnostics.Debugger.Break();

A breakpoint does not occur.

Therefore, I think this is due to the fact that IIS cannot compile the function in debug mode.

I added this to web.config, but this does not solve the problem:

<compilation debug="true"></compilation>

Sure, someone else has come across this, or is it just my specific computer error.

Thank! - Brian