[plt-scheme] DrScheme Can't Insert Negative Fractions?

This message was originally posted here:
http://groups.google.com/group/study-htdp/browse_thread/thread/4be081a64769dcfb
by my friend Kartik.
I cross-posted it on his behalf so that he might have an answer in the
morning (his time). BTW, we haven't discussed check-result and
check-within, yet.
;;
While attempting to solve fahrenheit to celsius problem I was trying
to add a simple test which tests the following condition

(= (fahrenheit-to-celsius 0) -17 7/9)

DrScheme allows adding fractions via Menu->Special->Insert Fraction.
However, when I try inserting a fraction for -17 7/9 it gets converted
to -16 2/9.

Is this a bug?  17 7/9 by itself gets added properly but not its
equivalent negative value.