mysqli_stmt::bind_param(), Number of variables doesn't match number of parameters in prepared statement

I wanted to use the prepared statements of mysqli, but the following problem occured: If I just use one parameter everything works fine $prep = $this->mysqli->prepare('INSERT INTO guestbook (Von, Datum) VALUES (?, NOW())'); $prep->bind_param('s', $this->Von); but if I try to use two $prep = $this->mysqli->prepare('INSERT INTO guestbook (Von, Betreff, Datum) VALUES (?, ?, NOW())'); echo $prep->param_count; // echos 2 $prep->bind_param('ss', $this->Von, $this->Betreff); I get an error Warning: mysqli_stmt::bind_param() : Number of variables doesn't match number of parameters in prepared statement in ... Am I doing something wrong or is it a bug or whatever? php version is 5.1.4 on winXP Best regards, Ulrich