AS3: hitTestObject turning up null for no apparent reason

I'm currently coding a game, and in one part of the code, it's checking to see if the player is close enough to a monster in mArray for its health bar to appear. I use hitTestObject for this with var i incrementing to go through the list. However, I will randomly get this error message:

TypeError: Error #2007: Parameter hitTestObject must be non-null.

After seeing this for the first time, I put the test all under the conditional if (mArray.length > 0) to make sure the Array() was filled before even attempting the test... but it still occurs at random times. I even used trace(i) to see if it happened on a certain part of the list, but it's completely random each debug session! I know the player portion can't be null since it is on the screen at all times during the hitTest, so I'm at a loss for words right now.

The portion of code in question (nothing else is related to this except for the use of mArray[i] in different places):

for (var i:int = 0; i < mArray.length; i++) {
    if (mArray.length > 0) {

        trace(i); //my attempt to see if it occurred at a specific incrementation of i

        if (player.hitTestObject(mArray[i])) {
            mArray[i].healthBar.visible = true;
        } else {
            mArray[i].healthBar.visible = false;
        }

    }
}

Again, it works perfectly for everything all of the time except for random cases it gives me the TypeError. Is this just a hiccup in my code, is there some obvious problem that I'm overlooking, or does the code just like to throwback to 2007?

EDIT: The debug session ALWAYS points to the line with if (player.hitTestObject(mArray[i])).