AR Core Sdk Preview XCode build issue

I’m following the steps in the guide here: Guia de início rápido para o Unreal  |  ARCore  |  Google Developers

and attempting to compile from source as instructed here: Building Unreal Engine from Source | Unreal Engine Documentation

After running Setup and GenerateProjectFiles, I open it in XCode 9 and attempt to build. At the beginning of the build process, it fails early when running the command “UnrealBuildTool.exe UE4Editor Mac Development”, saying that it can’t resolve the method “Array.Empty”. I have verified that I have the mono framework installed, version What’s interesting is that if I run that same command from the terminal, the build succeeds. Does anyone know what is going on in XCode that causes this failure? Is it trying to use the wrong version of Mono?

Thanks in advance.