FEPEST error 'Missing matrix column'

Hi all.

I am trying to calibrate a 3D unconfined aquifer system using FEPEST and found that the model ended with error message below:

'[ERROR] Missing matrix column: 400|27440.98^7807.29^10.00^-1'

Can anyone explain what sort of problem my model has and how I can fix it?

Thank you in advance. 

Hi Lee,
I suspect that the observation point is located in an inactive element?
Could you please check?
Best,