Quick Question: Simple Homebrew Feat, But Cannot Get Bonuses Correct

I have created a homebrew initiative feat that all characters and NPCs in my campaign receive. It is called Realistic Initiative.

This is a fix for the semi-broken initiative system

• You use the sum of your Dexterity modifier and your Wisdom modifier for initiative.

Under the "Modifiers" section, I have chosen

Modifier: Bonus

Modifier Subtype: Initiative

Ability Score: WIS

This works beautifully for wisdom bonuses but does not subtract from initiative if the WIS modifier is negative. Any idea how to get this to work?

Thanks ahead of time,

Roy