The stress-strain matrix is not positive definite?

The stress-strain matrix of material 1 is not positive definite, which  is required for real materials.  Being positive definite means that    
1.0 - NUXY**2*EX/EY - NUYZ**2*EY/EZ - NUXZ**2*EX/EZ -                  
 2.0*NUXY*NUYZ*NUXZ*EX/EZ must be positive, but is equal to             
 -495237.185.  Consider reducing the Poisson's ratios.  请问高手这是什么原因？材性是别人用过的，应该不会错，求救!