Optistruct internal programing error 8006

Hi, 

I am running  optistuct  random  frequency response. Applying PSD load  input. 

I am getting error 8006. 

 *** INTERNAL PROGRAMMING ERROR # 8006 ***
 Data block isubra not in data manager dictionary.
 * DATAMANAGER ERROR in subroutine 'locateit'.
 ****  Error 8006 in datamr() during 'locate' operation.

Can you help me in this?