Warning: Cannot use a scalar value as an array [ 2.0 版本 ]

Warning: Cannot use a scalar value as an array，请问一下这是怎么回事呢？

<?php
$b = [];
$a = array(1,31,45,56,78,84,93,107,154);

foreach($a as $k=>$v){
	$a[$k][]['num'] = 1;
}
print_r($a);