wp_login returns error

I put <?php wp_login(); ?> in my template header, and I am getting this error message:

Warning: Missing argument 1 for wp_login() in /home/blueribb/public_html/wp-includes/pluggable.php on line 170

Warning: Missing argument 2 for wp_login() in /home/blueribb/public_html/wp-includes/pluggable.php on line 170

Here is the function itself, starting at line 170:

function wp_login($username, $password, $already_md5 = false) {
	global $wpdb, $error;

	if ( '' == $username )
		return false;

	if ( '' == $password ) {
		$error = __('<strong>ERROR</strong>: The password field is empty.');
		return false;
	}

	$login = get_userdatabylogin($username);
	//$login = $wpdb->get_row("SELECT ID, user_login, user_pass FROM $wpdb->users WHERE user_login = '$username'");

	if (!$login) {
		$error = __('<strong>ERROR</strong>: Invalid username.');
		return false;
	} else {
		// If the password is already_md5, it has been double hashed.
		// Otherwise, it is plain text.
		if ( ($already_md5 && md5($login->user_pass) == $password) || ($login->user_login == $username && $login->user_pass == md5($password)) ) {
			return true;
		} else {
			$error = __('<strong>ERROR</strong>: Incorrect password.');
			$pwd = '';
			return false;
		}
	}
}
endif;

Any thoughts as to why I’m getting this error? I put the function call for wp_register right before that, and it comes out just fine.

Thx.