Graph theory in conn (NaN value)

From the graph theory page,
i use the conn toolbox to explore RS functional connectivity between a set of 2 groups.
but when I finished set up(subject effect, [1,-1]..),
i get NaN values for analysis results in bottom of second-level results tab.
and then, i get no visual data and any values in graph theory. i couldn't see any node and values in table.
This does not seem correct. Can someone please explain this to me?
(I do not have this problem if I select just one group. this ploblem only showed when i tried two-sample t-test).