/usr/bin/which returns cryptic error "which: no ls in ((null))"

I am writing a simple shell program. When I use /usr/bin/which with my shell in mac osx and ubuntu, it acts normally. When I use the exact same command on Red Hat Enterprise Linux Client release 6.3 (Santiago), I get this error: "which: no ANYCOMMANDHERE in ((null))". Is there any intuition for this? I can't even find what the error means (Let me know if showing my source will help). **EDIT: My path is (from inside the shell):** $ echo $PATH /usr/lib64/qt-3.3/bin:/usr/local/bin:/bin:/usr/bin Thanks, Jon