Interesting bugs - MSB3246: Resolved file has a bad image, no metadata, or is otherwise inaccessible. Image is too small.

I got a very strange warning recently when building a .NET Core app with "dotnet build." MSB3246: Resolved file has a bad image, no metadata, or is otherwise inaccessible.Image is too small. Eek! It's clear, in that something is "too small" but what? A file I guess? Maybe it's the wrong size?