On my development system, a subscript out of range problem seems to return the NaN value rather than a runtime error. This is also true when running a binary on the same system. When I distribute the binary to a user who runs it under the VM, the subscript problem throws a runtime exception and terminates the program. How do I get my development system to throw runtime exceptions in this circumstance? I need to find these problems myself so my users don't. I am running IDL 8.2.1 Build id: 20120820 Thanks!