Bad material definition: vumat

Hi,

I am simulating a sphere inclusion in a cube. The cube and the
spherical inclusion have different material properties and their
constitutive behavior follows the same equations.  I use vumat to
describe the constitutive behavior. The material properties
and the vumat have been tested individually and they work fine.

When, I use two different sets of material properties simultaneously in simulation, I get the following error

***ERROR: Bad Material definition in element number 136 instance
MERGE-1: zero or negative initial dilatational modulus caused by bad
material data. Please check your material input and any initial
conditions if necessary.

The dilatation modulus (= (stressNew-stressOld)/strainInc for both sets
material parameters at t=0 is greater than zero. So i am not really
sure, why this error would occur.

I would really appreciate inputs to tackle this problem.

Thanks