Rigify Generate Rig: index out of range

I’m using rigify to rig a character I’ve setup, the weight paint and movement all works fine but rigify is generating an error "Generation has thrown an exception: list index out of range" when I click generate rig. I’ve included a link to the blend file if that helps as I can’t figure out whats causing the error and haven’t been able to find an answer on Google. I’ve gone cleaned up my model and had a look through the bones but can’t find any issues. I have changed some bones from the default rig if that’s an issue so not sure if I need to do something extra to update rigify. Any help would be greatly appreciated!

https://drive.google.com/file/d/1oVaTbJNIyH9ekEXSVOAklCkZe1uhcKSn/view?usp=sharing