How do I fill a uniform float3x3?

How does one set a uniform 3x3 shader matrix from script? I’ve tried

Matrix4x4 myMatrix;
Shader.SetGlobalMatrix("myUniform", myMatrix);

but the compiler comes back with:
Shader error in ‘Marmoset/Specular IBL’: Fragment program ‘frag_MarmosetIBL’: non-4 sized matrix myUniform at line 19

Does shaderLab even allow for “non-4 sized” matrices?

Currently it does not :expressionless: