Error simulating fluid flow through microsieve

Hi everyone,
I'm newbie to COMSOL Multiphysics.
I'm trying to anticipate when a microsieve is broken under fluid flow (blood filtering), but some errors keep occuring.
Please help me!
Thanks very much!

[File]
drive.google.com/file/d/0B15nm...JZSnZodWtEMFU/edit?usp=sharing

[Error]
Failed to find a solution.
Segregated group 1

Singular matrix.

There are 1 void equations (empty rows in matrix) for the variable mod1.fsi.Pmfinl1.
at coordinates: (0,0,0), ...
and similarly for the degrees of freedom (empty columns in matrix).
Returned solution is not converged.