What makes a Container Image scan 'invalid'?

I'm trying to scan container images in Cloud Optix from an ECR registry. However in the scan queue they are all listed as 'invalid'. The documentation states:

"If an image scan request is invalid, its status is shown as Invalid and the scan request is cancelled."

That's fine, but what would cause an invalid scan request? Can't seem to find anything within the documentation beyond what I've stated above....