How would a tunneled Excel refnum go from a valid value to a NULL between iterations of a For Loop? For some reason this works find in one VI and when executed and not in another. Here's screen shots of the highlighted execution with probes appropriately. The "Open" refnum is passed into "Performance Test - Write Data To Files.vi". That's where the trouble is. You can see that it goes into the For Loop. The first iteration is okay, but the second and subsequent iterations have the refnum NULL. See image below.