json_encode returning null if array not accessed before

In my code I build an array and encode it using json_encode, json_encode returns null for this array unless I insert the instruction "echo $responce->rows[0][0];" before encoding, if I comment out this row json_encode returns null!

Can someone find out why?

echo $responce->rows[0][0];
echo json_encode($responce);

It has this behaviour with this encoded json, but works for different arrays:

{"rows":[{"id":"33UD","cell":["Great Yarmouth Borough Council",5875732.23,61.01]},
{"id":"41U开发者_如何学PythonE","cell":["Newcastle-Under-Lyme District Council",2514111.76,20.24]},
{"id":"36UF","cell":["Ryedale District Council",96439.18,1.8]},{"id":"00CM","cell":
["Sunderland City Council",16473262.71,58.48]}]}

You will get null returned by json_encode if your array values are not encoded to utf8 (so they are not safe for json_encode)

if you get your data from a db try using:

mysql_query('SET CHARACTER SET utf8') before your SELECT statement