valgrind: command not found (even when valgrind is available)

When I try to run Valgrind Function Profiler, it works fine in 2.5.2, but in 2.6 it says "valgrind: : command not found". 'which valgrind' from a terminal shows that valgrind is indeed on PATH, and since Valgrind Function Profiler still works in 2.5.2 on the same system I don't think that a missing valgrind is the problem. What else could be wrong?

(Note that it worked one time after I built 2.6, but I cannot get it to work again).