I never thought about it until you mentioned it, so I did not know this before looking at the instructions. Gossen states (I assume they make their meters to conform to ISO standards, IDK) the sensitivity of my Luna Pro F at -3 to +15 EV at ISO 25----so at two stops slower film rating, a -3 EV will give an 8hr exposure at f/64.
I would need two stops more light value (ISO 100) for a reading at -3 EV to accomplish the same exposure of 8hrs at f/64.
This means, my meter tells me, and if I'm thinking correctly, that my Gossen will effectively read a -5 EV at ISO 100 and not exceed it's sensitivity rating. A lower EV should be able to be accurately read if the ISO is higher than the accurate minimum ISO stated by the manufacturer---------I think.
Even so, I will probably not ever use this meter in that manner, but either way, it's still a pretty darn low light level for these meters to be able to read and the OP couldn't go wrong with them.