'Invalid option '9.0' for /langversion

I'm receiving the following when I attempt to build my Unity project.

It builds in Unity without complaint, but fails in Rider when I attempt to rebuild solution (previously it didn't).

Versions Unity:2021.2.8f1 Odin:3.0.12 JB Rider: 2021.3.3

Sirenix.OdinInspector.CompatibilityLayer
  Invalid option '9.0' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default

Sirenix.OdinInspector.UnityMathematics
  Invalid option '9.0' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default
c++sharpunity3d


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