mysqli - fetch_Array 错误调用非对象 mysqli 上的成员函数 fetch_array()

I am new to mysqli and started trying to learn basic things. With respect to this i example (http://php.net/manual/en/mysqli-result.fetch-array.php) i was trying fetch_array. Here is my code.

$sqlGetChartData    =   "SELECT date, ratepersqft, location 
                          FROM ratepersqft
                         WHERE project_id = 1";
$runGetChartData    =   $mysqli->query($sqlGetChartData);

while($rowGetChartData = $runGetChartData->fetch_array(MYSQLI_BOTH))
    $arrGetChartData[]  =   $rowGetChartData;

    print "<pre>";
    print_r($arrGetChartData);
    exit();

这里我收到此错误 Call to a member function fetch_array() on an non-object on a non-object on line next to while condition line.我尝试使用谷歌搜索它并没有得到我的问题的结果.希望我的问题很清楚.提前致谢.