[NTG-context] indexing query

I am trying to construct an index of passages and have run into a curious problem: The test file is: \defineregister[Passage][Passages] \setupregister[Passage][n=1,% balance=yes,% pagestyle=\tfx\em,% textstyle=\tfx,% criterium=all,% interaction={pagenumber},% ] \setupregister[Passage][indicator=no,distance=0.5em] \starttext See \Passage{Simplicius+{\em In de caelo}+483.15--19}483.15--19. \page See \Passage{Simplicius+{\em In de caelo}+477.3--4}477.3--4 \page \placeregister[Passage] \stoptext The result is an index that puts 483.15–19 *before* 477.3–4 as in Simplicius In de caelo 483.15–19 1 477.3–4 2 rather than after. How can I fix this? Alan