Comparator flashes off/on when entering the world

I have a comparator attached to a command block that provides a signal strength, and when I exit then re-enter the world, the comparator flashes off then on once, executing other commands when I don't want them to.
The command block is updated using a regular repeater clock, or using setblock ~ ~1 ~ redstone_block 0 destroy as the clock.

This also happens when accessing the command block then hitting done.

The command being used is:

testfor @e[type=Player,score_ScoreA=0,score_ScoreA_min=0]