Apple is warning of an AAC problem that may affect its new iPod shuffle.

A technological note posted on the company's support Web site reveals that some AAC files that were not encoded using iTunes may not play on the tiny new music player.

"If you try to play a song on iPod shuffle that was encoded in AAC format (.M4A) by an application other than iTunes, iPod shuffle may not play it and skip to the next song. To prevent this from happening, always use iTunes to encode songs to AAC for iPod shuffle play," Apple advises.

Songs purchased from the iTunes Music Store work fine, Apple says.

The .M4A format is supported within QuickTime, iTunes, foobar, 3ivx, Winamp and, of course, Real Player. RealNetworks Rhapsody service offers songs for sale in AAC format.