PHP Fatal error: Call to a member function num_rows() on boolean

PHP Code:

public function count_all_results($table = '', $reset = TRUE)
{
if ($table !== '')
{
$this->_track_aliases($table);
$this->from($table);
}

$result = ($this->qb_distinct === TRUE)
? $this->query($this->_count_string.$this->protect_identifiers('numrows')."\nFROM (\n".$this->_compile_select()."\n) CI_count_all_results")
: $this->query($this->_compile_select($this->_count_string.$this->protect_identifiers('numrows')));

if ($reset === TRUE)
{
$this->_reset_select();
}

if ($result->num_rows() === 0) // the error is on this line
{
return 0;
}

$row = $result->row();
return (int) $row->numrows;
} 


// I have searched for a solution but nothing so far