Problem in 1D EHD simulation

Dear all,

I have built a 1D EHD in COMSOL. But it does not work. There always exists an error. Can anybody give me any suggestion? Please help me! My *.mph file is in the attachment. The error is as follows

The following feature has encountered a problem:
Failed to find a solution.
Singular matrix.
There are 249511 void equations (empty rows in matrix) for the variable comp1.P.
at coordinates: (2.5949,0), (2.5948,0), (2.59482,0), (2.59484,0), (2.59486,0), ...
There are 1 degrees of freedom that do not occur in any equation (empty columns in matrix) for the variable comp1.H0.
at coordinates: (0,0), ...
Returned solution is not converged.
- Feature: Stationary Solver 1 (sol1/s1)

Thanks a lot!