"Build failed. Path is empty" error after upgrading to XS 4.2.3 (build 51) and iOS 7.0.6.168

After upgrading to the latest beta bits this morning, I'm unable to build my iOS app. I'm getting the following vague error:

---------------------- Done ----------------------

Build failed. Path is empty
Build: 1 error, 0 warnings

Any suggestions? I'm running Xamarin Studio 4.2.3 (build 51) and iOS 7.0.6.168 on OS X.