Invalid Protected Resource URL in Pecl_Oauth

I had a funny (funny weird, not funny haha) problem the other day when working with pecl_oauth in PHP to talk to a service. I'd gone through all the handshaking steps, got the acces token and was ready to start talking to the service itself. However when I tried to call OAuth::fetch, I got an error message:

Fatal error: Uncaught exception 'OAuthException' with message 'Invalid protected resource url, unable to generate signature base string'

There are two things to notice about this. The first one is that I should be catching exceptions thrown by this code :) The second is that I could see nothing wrong with my url, http://api.local. It turned out, after some experimentation, that what is missing here is a trailing slash, and if I supply http://api.local/, everything works perfectly nicely! I'm unclear if this is intended functionality or not, but if you see this error message and you're requesting a URL with no path info, make sure you have a trailing slash.