NL50: Pr'd Board on River w/ Straight

[oakrdrzfan]

Villain bet small on flop and turn and now donks when the board pairs the river. Is this a push or should i be supsect and just call. No history with villain.

Full Tilt Poker, $0.25/$0.50 NL Hold'em Cash Game, 8 Players
LeggoPoker.com - Hand History Converter

CO: $50
Hero (BTN): $62.50
SB: $49.50
BB: $47
UTG: $28.75
UTG+1: $55.85
MP1: $24.05
MP2: $49.20

CO posts $0.50
Pre-Flop: 4[img]/images/graemlins/heart.gif[/img] 6[img]/images/graemlins/heart.gif[/img] dealt to Hero (BTN)
4 folds, <font color="red">CO raises to $1</font>, Hero calls $1, SB folds, BB calls $0.50

Flop: ($3.25) 5[img]/images/graemlins/spade.gif[/img] 7[img]/images/graemlins/club.gif[/img] K[img]/images/graemlins/spade.gif[/img] (3 Players)
BB checks, <font color="red">CO bets $1.50</font>, Hero calls $1.50, BB folds

Turn: ($6.25) 8[img]/images/graemlins/heart.gif[/img] (2 Players)
<font color="red">CO bets $2.50</font>, <font color="red">Hero raises to $13.75</font>, CO calls $11.25

River: ($33.75) 8[img]/images/graemlins/diamond.gif[/img] (2 Players)
<font color="red">CO bets $2.50</font>, Hero ???