RouteCalculation失败，错误代码为28（SKRoutingErrorCodeInvalidNumberOfRoutes）

Question

我编写了以下用于计算离线skmaps中路线的代码。开始和结束坐标是正确的，但路线计算失败，错误代码（ SKRoutingErrorCodeInvalidNumberOfRoute ）。我也试过设置route.maximumReturnedRoutes = 3，但什么也没发生。

-(void)FindRoute
{
    [SKRoutingService sharedInstance].routingDelegate = self;
    [SKRoutingService sharedInstance].mapView = self.mapView;
    route.startCoordinate = StartCoordinate;
    route.destinationCoordinate = EndCoordinate;
    route.shouldBeRendered = YES;
    route.requestAdvices = YES;
    route.maximumReturnedRoutes = 1;
    route.requestExtendedRoutePointsInfo = NO;
    [[SKRoutingService sharedInstance] calculateRoute:route];
}