Build fails on .NET 4.6.1 project using new csproj sdk format

Specifically: /usr/lib/mono/msbuild/Current/bin/Microsoft.Common.CurrentVersion.targets(2101,5): error MSB3248: Parameter "AssemblyFiles" has invalid value "/usr/lib/mono/4.6.1-api/mscorlib.dll". Could not load file or assembly 'System.Reflection.Metadata, Version=, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. [/home/travis/build/projectcomet64/M64MM/M64MM.Addon/M64MM.Addon.csproj]

The project used to build correctly, but now it doesn’t after it failed for the first time (because of an addition that had a case-sensitivity error)

This, somehow, was fixed by re-reverting changes to that project. Now it builds. Strange.