Creating a new Sketch plane on a rib?

Hello,

I am working on one of the tutorials - Creating Draft, Shell, and Rib Reatures, and building the Step part.  At one point the tutorial wants you to select a previously created rib as a sketch plane, but it will not let me select the rib.  I can choose any other face, but not the rib.  Anyone have any suggestions?

Thanks in advance!