Hum with 2 pickups out of phase

[Peter Z]

I installed a Schaten HFN pickup in a Gibson L4a with a built in Prefix plus preamp. Quite good but not perfect.

Next I installed the existing UST in parallel. Even better but it seems there is a small problem with the phase because the low E string is a little week.

So i tried to install the UST out of phase. Sound is perfect but now I get a inacceptable loud hum.

Any idea what is the problem here? Why does the humming only occur when the pickup is installed out of phase and not with one of the single pickups or both installed in phase?

[jonfields45]

The shield of the UST is no longer attached to electrical ground.

[Peter Z]

Of course!!!!!
My dear, I didn't think about this. Thank you, Jon!