incompressible-plane-stress

 I am mohammad , I am using VUMAT to implement a 2D incompressible
hyperelastic model for red blood cell , I do have a couple of questions
In plane stress( shell ) in abaqus formulation , the first and second
invariant of Right Cauchy green strain tensor are given by
 I_1 = lambda1^2 + lambda2^2
I_2= lambda1^2*lambda2^2
 Is that right ?

Another thing , when I define the hydrostatic pressure from stress boundary
condition (sig33=0) , abaqus give me an error initial negative dilational
modulus , when I flip the sign or remove the hydrostatic pressure term it
works , so does the pressure stress affect the deformation  ? ,or the
hydrostatic term to keep material incomressibiity and keep solution stable
, is it necessary to include.

By the way I return the strainINC (I,3)=0 , as I want  to keep membrane
thickness constant

Regards
Mohammad