abaqus求解自由模态报错，请问什么问题引起的？ 20

大神好。

abaqus求解自由模态，

报以下错误，请问是什么问题呢？怎么修改？

 ***ERROR: LANCZOS EIGENSOLVER ENCOUNTERED NUMERICAL SINGULARITY OR ZERO PIVOT. 

        THIS MAY HAPPEN IF A BOUNDARY OF THE COMPUTATIONAL INTERVAL IS VERY 

        CLOSE TO AN EIGENVALUE, THE PROBLEM HAS AN UNCONNECTED UNCONSTRAINED 

        REGION WITH ZERO MASS MATRIX, OR A SYSTEM IS OVERCONSTRAINED.