Pull Down on Comparator Input
LM139 is a ruggedised version of LM339 a very common comparator.

This part has a PNP input stage, and an open circuited pin will float high in the absence of a pull down.

The data sheet says the bias current is up-to 100nA (at room temperature), so with a 1M pull down there could be 0.1V on the input pin. (at worst case allowed operating temperature it could be 300nA, and 0.3V)

The other input will have about 0.16V and given that the offset voltage (imprecision) is only 0.002V it should work (at room temp).