Can't navigate to declaration on Xamarin classes

Whenever I attempt to go to declaration on a Xamarin Forms class (e.g. Button, Label, TapGestureRecognizer, Page), I always get a dialog saying an error occurred, and if I view details it says "System.AggregateException: One or more errors occurred"

When I dismiss the dialog it just takes me to the Assembly Browser root file. I clicked Always send this error to Xamarin but don't know if that is monitored.

I think this once worked, when I first started using Xamarin Studio, but now it does not. It is Xamarin Studio on the Mac, if that matters. Anyone else see this?