Hero ($33) BTN AhKh 50c
BB 3bet $1.80
Hero 4bet $5.80
BB ($40+) calls

Flop ($11.7) Kd Qd 9s
BB leads $6.50
Hero calls

Turn ($24.7) Kd Qd 9s 6c
BB all in
Hero ($21) AhKh ??

BB is typically a nitty player. Has a limping range preflop. Have seen him check down TP IP for 3 streets HU. Feels like I should fold, but don't think I'm supposed to without a diamond.