Strange "trying to get property of non-object" error

[eluser]Loquela[/eluser]
If it helps a var_dump of the object $topics gives:

Code:

array(3) {
["subject_id"]=>  
string(2) "17"
["subjects"]=>  array(8) {
[0]=>  object(stdClass)#18 (2) {
["subject_id"]=>  string(2) "21"
["subject"]=>  string(23) "Fairy tales and stories" }
[1]=>  object(stdClass)#19 (2) {
["subject_id"]=>  string(2) "17"
["subject"]=>  string(9) "Geography" }
[2]=>  object(stdClass)#20 (2) {
["subject_id"]=>  string(2) "16"
["subject"]=>  string(7) "History" }
[3]=>  object(stdClass)#21 (2) {
["subject_id"]=>  string(2) "25"
["subject"]=>  string(27) "Personal, social and health" }
[4]=>  object(stdClass)#22 (2) {
["subject_id"]=>  string(2) "34"
["subject"]=>  string(8) "Religion" }
[5]=>  object(stdClass)#23 (2) {
["subject_id"]=>  string(2) "15"
["subject"]=>  string(7) "Science" }
[6]=>  object(stdClass)#24 (2) {
["subject_id"]=>  string(2) "33"
["subject"]=>  string(12) "TEST SUBJECT" }
[7]=>  object(stdClass)#25 (2) {
["subject_id"]=>  string(2) "35"
["subject"]=>  string(6) "Tester" } }
[0]=>  object(stdClass)#26 (3) {
["topic_id"]=>  string(1) "5"
["topic"]=>  string(6) "Forces"
["subject_id"]=>  string(2) "17" } }