Problem with Print Booklet in InDesign

I'm trying to export a PostScript file of a book design in InDesign with pages arranged for a saddle-stitched book using the Print Booklet feature. The problem i'm having is this:

Print Booklet shows a warning saying that the booklet doesn't fir the current paper size. The preview shows the landscape design rotated to a portrait orientation.

The page origination in Document Setup and the design itself is correct:

Any ideas as to the cause of the problem?