triggers E_ERROR: "Cannot call overloaded function for non-object" on line 13

gistfile1.php

<?php
private function getKey() {
  if ($this->foo) {
    $config = get_config();
    if (!isset($config['func']) || !is_callable($config['func'])) {
      throw new Exception("Function missing")
    }
    $func = $config['func'];
  }
  else {
    $func = function(model $m) { /* do some stuff */ };
  }
  return $func($this->m);
}

the closure on line 11 has the same signature as the callable returned in the config and set on line 8, of course.