错误：脚本标头过早结束：php5

Question

When I run the following

foreach($csv as $row) {
if (!in_array($row[2], $institutions)) {
  $institutions[$row[2]] = $row;
}

I get no errors, but when I run this

foreach($csv as $row) {
if (!in_array($row[2], $institutions)) {
  $institutions[$row[2]][] = $row;
}
}

I get this error

Premature end of script headers: php5

What is wrong with my code? What I'm trying to do is store all rows from the $csv array into subarrays, with the key being the institution name.