Why array_diff() gives Array to string conversion error?

I get array to string conversion error for the following line:

$diff = array_diff($stockist, $arr);

Here, $arr is an array decoded from a JSON file. Using the is_array() function I was able to verify that both parameters are arrays. Can someone point me the problem

    $stockist = array();
   while (!feof($file_handle) ) 
   {
    $line_of_text = fgetcsv($file_handle);

    $query = "select * from reorderchart where medicine='".trim($line_of_text[3])."' ORDER BY medicine";

    $result = mysql_query($query);


      if(   trim($line_of_text[2])  -  trim($line_of_text[1]) <= 0 )
      {
      while($row = mysql_fetch_array($result))
         {   

            $file = "results.json";

            $arr = json_decode(file_get_contents($file),true);
            $pharmacy = trim($row['Medicine']);

            if(isset($stockist[$pharmacy]))
             {
              $medicine = $stockist[$pharmacy];
              $medicine[] = trim($row['Stockist']);
              $stockist[$pharmacy] = $medicine;
             }
            else
            {
             $medicine = array();
             $medicine[] = trim($row['Stockist']);
             $stockist[$pharmacy] = $medicine;

            }



         }

}

     }
     $diff = array();
     $diff = array_diff_assoc($stockist,$arr);
     ksort($diff);
        foreach ($diff as $key => $value) 
        {
        echo "<table align='center' border='1'>";
        echo "<tr><td align = 'center'> <font color = 'blue'> $key</td></tr>";
            foreach($value as $key1 => $value1)
              {
              echo "<tr><td align ='center'>$value1</td></tr><br>";
              }
        echo "</table>";    
        }