about 'Warning : Re-ordering of null vectors failed"

Dear all,

I'm trying to simulate the dynamic behaviors of the linear control solenoid valve.
The spool motion is determined by the force balances, which are calculated from each field, and a spring-mass-damper system.
So, I am using the AC/DC module and Creeping flow with the ODE module for ALE.

When the spool in the valve starts to move, I got a message ' Warning: Re-ordering of null vectors failed".

I want to know what the meaning of the warning message is.

Does anybody have any advice?

Regards,
Lee.