I am using PSSE 34.9.5, I did perform qv analysis successfully for all the buses of interest except for one that I am getting the following IERR=8

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I am using PSSE 34.9.5, I did perform qv analysis successfully for all the buses of interest except for one that I am getting the following IERR=8

Messages for api QVENGINE4 Invalid study bus 20002 [5ROB275A275.00]: (003286) Its voltage is controlled by generation at bus 20333 [5ROB_SC115.000] (003306)

Does anybody know what 'invalid bus study' means here? And why, when I follow the same sequence/approach for another case with exactly the same buses but with just a different dispatch, it runs smoothly without any trouble! Your help is greatly appreciated.