river play with bottom 2

[gnads]

Villain was loose passive, 34/5/0.37. The whole table was passive so I had started raising light in the last few orbits

What's our river play in this spot?

PokerStars No-Limit Hold'em, $0.25 BB (6 handed)

Hero ($43.35)
Button ($27.05)
SB ($22.90)
BB ($23.40)
UTG ($7.20)
MP ($28.10)

Preflop: Hero is CO with 4, 5.
2 folds, Hero raises to $1, Button calls $1, SB calls $0.90, 1 fold.

Flop: ($3.25) K, 5, 4 (3 players)
SB checks, Hero bets $2.75, Button calls $2.75, SB folds.

Turn: ($8.75) A (2 players)
Hero bets $5, Button calls $5.

River: ($18.75) 9 (2 players)
Hero ???

[freechus9]

I'd block bet about 7-8 and fold to a raise. That flush scares me.