Normal flux problem of transport of diluted species

Hi there,
I am solving possoin-nernst-planck equations using comsol.
When I combine Electrostatic module and transport of diluted species module together, I apply posive voltage and negative voltage on two opposite sides in es module and apply normal zero flux in transport of diluted species module. I use time dependent solver. Though I can get solution, I found normal flux is not zero at the boundary. I am so confused about this result since I predifined that.

A lot of thanks in advance.
Best,
Leo