A new value was added to the FailureErrorCode diagnostic, and this will now throw: https://github.com/dotnet/roslyn/blob/bd3a0b49d72f6c2b0b00e6fbe8cef5a2b85ee3b3/src/Features/Core/Portable/Diagnostics/AnalyzerHelper.cs#L206-L207 I'm not sure what the impact of that is yet.