Interesting Limit Holdem River Decision

I played a little B&M this weekend for a small profit and in the course was involved in a somewhat interesting river decision.

I raised 2 loose limpers with black K's and 4 of us saw a flop of Kd-4d-5c. I bet and got 2 callers. The turn was the 9s and I bet again and got 1 caller. The river was a scare card, the 6d for a board of Kd-4d-5c-9s-6d and now I get bet into! Despite this player being very donkish, the correct decision here is to just call. Why? Because if you raise and are ahead you gain either 0 or 1 additional bet whereas if you are behind you'll likely get re-raised and lose 2 additional bets. In the hand I just called and the donkey was bluffing with QJo. Had I raised he would have folded.