You don't have to worry about it.
The error only means that avast! didn't scan the content of the (unpacked) file because it wasn't able to unpack it (but the file still was scanned from the "outside").
Why avast! wasn't able to unpack the file, that's a question... either the file is really corrupted, or it's a format/version unsupported by avast! currently. I'm afraid I can't say more without taking a look at that particular file, if you'd be able to send it somehow.