Question ADB backup create empty backup

Hi I want create backup of "Google music" include cached music.
So I use command
adb backup -f music.ab
I was confirm on screen, and waitning (3 sec) for complete backup.

Backup file has 41b size and inside is 5 lines.
Where is problem?
Can someone try make backup of this app too?

Backup od works fine.
Thnak you

Phone: ParanoidAndroid 4.5b3
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