can't use $_GET with language in clean-urls

On one page i use

header('Location: contact/error');
exit();

and this works fine, i can use the $_GET['error'] but when i add the language like

header('Location: en/contact/error');
exit();

it doesn't work anymore, it will give an undefined index

my htaccess looks like this

RewriteRule ^en/contact/error contact.php?error=1&lang=en [L]

i don't understand why it doesn't work, can someone please help me?