C#: syntax error at token '-' in:IL_0029: ldc.r8-inf when building from source

Question

I am trying to contribute to an open source project open source project. I've never worked with C# before, so I am not sure exactly how to write this out, but, here is what I've done so far.

I downloaded Visual Studio and some workloads, then forked and cloned the repo, and made a new project with the repo's .sln file. The project requires some API keys, so I filled those in, and then tried building.

I get the following error:

error : syntax error at token '-' in:     IL_0029:  ldc.r8     -inf

Google hasn't helped. I found this, but have no idea what it means.