Presumably the value for subkeyname being passed to OpenKey contains an embedded null, which I believe is legal for the registry in general, but doesn't make much sense in this context and is quite possibly a corruption issue on your machine.

We can certainly strengthen mimetypes against this error (I don't know who's maintaining this module, but handling the TypeError on OpenKey seems safe enough) and you may also want to ensure your virus/malware scanners are up to date - embedded nulls in registry keys are often used to trigger bugs or hide malicious data.