Abstract generation : Warning : Design contain zero-pin terminal.

I'm using Virtuoso Abstract Generator , version  IC6.1.6-64b.500.14 . When trying to run step pins for the cells, I got this warning:

> *WARNING* (ABS-12042): Cell NAND2X1: Design contains zero-pin terminals. There terminals will be deleted.

>INFO (ABS-12001): Cell NAND2X1: There are 5 existing terminals.No pins will be added for them.

>INFO (ABS-11901): Cell NAND2X1: step pin finished.

and pin checking is not successful. I checked the design couple of time and all the pins have proper terminal and pin name. I also generate abstract for INVX0 and INVX1 cells and they work out just fine. 

What could be the possible mistakes here?  How could I solve this? 

Thank You