Capabilities.screenDPI produces PackageInspectionFailed

I keep getting PackageInspectionFailed on a barebones test app using FlashBuilder and AIR 3.9.

If I comment out this line, the app compiles and debugs on device perfectly:

trace( "Screen DPI: " + Capabilities.screenDPI );

Why would this affect PackageInspection (what the heck IS PackageInspection anyway) ?