Hmm, it sounds like your tester may have a voltage offset problem. If you are using a DMM you can reverse the leads. You get 2 different voltages, then add them together and that cancels out the offset. If it's inside your resistance tester then you might want to invest in a new tester.
That measurement taken literally means the Electra tube connectors have negative resistance. If that's true then expect a knock on your door and very serious discussions with the military. I personally could find some great uses for a negative resistor.
EDIT: Also, if your resistor is hot during the measurement, and the tester leads are touching different metals (say copper on one, steel on the other), you may be getting offset from a thermocouple effect. Just shooting from the hip here though.