ITunes :: Unable To Convert Few Lossless (ALAC) Files To AAC

I recently decided to sync my entire iTunes Library to my iPod instead of managing it all manually. I use the option "Convert higher bit rate songs to 256 kbps AAC", since most of my library consists of CD rips. At the end of each sync, I receive the following message: 
"iTunes could not copy "Stage Start" to the iPod because the file could not be converted" 

I took a look at the tracks in question, and couldn't find any good reason that they would not convert. They are ALAC files that I ripped from CD, and there are other tracks on the same album that converted with no problems. As a test, I changed the Import Settings to AAC@256kbps, right-clicked on the file and chose "Create AAC version", but received this message.

I'm not sure why it would say the file format is not valid, when it is lossless like all the rest and plays fine. I checked the file(s) in Finder and it has the .m4a extension like all the rest. The only thing different I noticed is that the album art thumbnail doesn't show up for these particular tracks.