Reflecting Pool null state

[FranAvalon]

If you cannot produce any type of mana with Reflecting Pool (for example if it's your only land), you should still be able to tap it as a mana ability (but it won't produce any mana because its type is undefined). (see rule 106.5)

It seems that the ability of Reflecting Pool is not considered "tapping for mana" for purposes of Overgrowth. I tapped Reflecting Pool with Overgrowth and only got one mana.