[PHP-DB] newbie PHP/MySQL question

[toylet]

A table with a column big5 char(2) not null primary key.

  $target->query("delete from canton");
  for ($ii=0; $ii<256; $ii++) {
    for ($jj=0; $jj<256; $jj++) {
      echo $ii .".". $jj . "\n";
      $query="insert into canton ( big5 ) values ( '"
      . mysql_real_escape_string(chr($ii).chr($jj))
      . "' )";
      $target->query($query);
    }
  }

The program died with this output:

0.92
0.93
0.94
0.95
0.96
0.97
Duplicate entry '' for key 1

The character strings are unique. Why did it find a duplicate at (0,97)?