Integration error in simulink

Hi, I'm doing servo valve modeling and always get this error. Couldn't figure out what's the problem: Derivative of state '1' in block 'NuzzleFlapper2stage01/Nozzel-Flapper Flow/Integrator Limited2' at time 2.3553214032604879E-15 is not finite. The simulation will be stopped. There may be a singularity in the solution. If not, try reducing the step size (either by reducing the fixed step size or by tightening the error tolerances) Any response and help? Thanks MS