Note in the following code from question/type/match/questiontype.php that the variable $match_ans can be undefined at line 631 if the test at line 623 fails. Should be a suitable else for the if at 623.

//Get the match_sub from backup_ids (for the answer)
623 if ($match_answer_id) { // only recode answer if not 0, not answered yet
624 if (!$match_ans = backup_getid($restore->backup_unique_code,"question_match_sub",$match_answer_id))

{ 625 echo 'Could not recode answer in question_match_sub '.$match_answer_id.'<br />'; 626 }

627 }
628
629 if ($match_que) {
630 //If the question hasn't response, it must be 0
631 if (!$match_ans and $match_answer_id == 0)

{ 632 $match_ans->new_id = 0; 633 }