problem in membrane deformation simulation under pressure

Hi. I am simulating a spherical membrane deformation under hydraulic pressure, in which the liquid push a thin layer of solid to have deformation. I have included physics of laminar flow and fsi. However, there is error message of

Nonlinear solver did not converge.
Time : 0.08
Segregated group 2

Attempt to evaluate real square root of negative number.
Function: sqrt
Last time step is not converged.

I have changed the step sizes and finer meshes which do not work. Hope that can have some advice. Thanks.