***ERROR: Bad Material definition: Zero or negative initial dilatational modulus caused by bad material data. Please check your.

Hi,

I'm conducting hypoplastic VUMAT model in abaqus. It always has the following problem. 

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

For the single layer,it works well,but this problem will appear for two layers, even if I chose the same parameters.

Hoping anyone who has the same problem or has already settled successfully could give me some advice.

Looking forward to discuss with you. Your help is greatly appreciated!  

Thanks ! 

Pan