Adjustment brush tool will only work if I apply to an object

I'm suddenly having trouble using the adjustment brush tool. I was using it earlier while working in this file with no issues but now I can't get it to cooperate and I don't understand why. 

I've double checked that I'm on the right layer but for some reason nothing happens when I try to use the regular brush to add to my mask. However it will work if I select the "Apply to Object" option. 

Any ideas why this is happening? I feel like I'm probably missing something obvious.