must use Theme.Compat for AppTheme's parent?

I'm a starter of Android dev. my app's minSDKVersion is 21, AppTheme's parent is set to 'android:Theme.Material.Light', MainActivity inherits from Activity.

I got an exception when add an item. It says 'java.lang.UnsupportedOperationException: Failed to resolve attribute at index 13: TypedValue{t=0x2/d=0x7f030131 a=-1}'.

I finally found a solution: change AppTheme's parent to 'Theme.Compat', and MainActivity's superclass to AppCompatActivity.

Is this the only way? I must use compat? and why? please help me. thanks!

Start by reproducing the reported exception with minSDKVersion 21, an AppTheme parent of android:Theme.Material.Light, and MainActivity extending Activity. Compare that setup with Theme.Compat and AppCompatActivity, then document which configuration is supported and why the attribute lookup succeeds.

Written by the indexing model from the issue text.