ios百度地图周边城市检索POI失败

以下是代码：
参照官方文档写的

    _poiSearch = [[BMKPoiSearch alloc]init];
    _poiSearch.delegate = self;

    BMKNearbySearchOption *nearBySearchOption = [[BMKNearbySearchOption alloc]init];
    nearBySearchOption.pageIndex = 0; //第几页
    nearBySearchOption.pageCapacity = 10;  //最多几页
    nearBySearchOption.location = CLLocationCoordinate2DMake(39.915,116.404);
    nearBySearchOption.keyword = @"小吃";   //检索关键字
    nearBySearchOption.radius = 50000; //检索范围 m
    BOOL flag = [_poiSearch poiSearchNearBy:nearBySearchOption];
    if(flag){
        NSLog(@"城市内检索发送成功");
    }else{
        NSLog(@"城市内检索发送失败");
    }

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