Neysys Ventilation - Run-time error '9' Subscript out of range

Hi,

I keep getting the error code "Run-time error '9' Subscript out of range" whenever I draw a vertical extract duct on one of my floors. I'm guessing there is an error with this particular floor. However, when I try to delete the floor, it seems to be taking ages. Do I just need to wait, or is the delete a floor function not working also?