Custom specular reflection cubemap script from Unity documentation throwing errors

Hi, I'm using dynamic skyboxes and trying to update the custom cubemap using this script from the Unity documentation.

https://docs.unity3d.com/ScriptReference/RenderSettings-customReflection.html

But I'm getting a weird error I can't solve:

Graphics.CopyTexture can only copy between same texture format groups (d3d11 base formats: src=26 dst=9)

Any clue how to solve this?