Problem deleting layers in stackup

Hi,

Got a 8 layer stackup PCB design in Allegro 17.2 and want to remove a few layers as they are no longer necsaary and this would reduce the cost of the PCB too.  However, when I try to remove a layer in the stackup manager (I have version 17.40.019), I get the following error:

"ERROR(SPCMGR-368): Could not delete layer "GND4" from application database."

Any suggestions?

Chris