Error: undefined symbol

if(VoziloJeAvion(GetVehicleModel(o)) || VoziloJeBrod(GetVehicleModel(o)) || VoziloJeKamion(GetVehicleModel(o))) Fuel = 80;

stock VoziloJeAvion(id)
{
	if(id == 592 || id == 577 || id == 511 || id == 512 || id == 593 || id == 520 || id == 553 || id == 476 || id == 519 || id == 460 || id == 513) return 1;
	else if(id == 548 || id == 425 || id == 417 || id == 487 || id == 488 || id == 497 || id == 563 || id == 447 || id == 469) return 1;
	else return 0;
}