Deleting a registry key

CCleaner for Windows CCleaner Windows Bug Reporting
1

I ran CCleaner and when scanning the registry, there was one issue whose suggested solution was: Delete Invalid DefaultIcon Key. I found it, tried to delete it, and some error came up saying there was one part it could not delete. So what do I do? :unsure:

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I ran CCleaner in the first place because I was unable to install an AVG update.