Sort with custom field in Galleries doesnt work

Hello, i want to sort my Pictures in my website. I cannot use the normal choices like Name, ID and so on. So i us the Custom field. The custom field has the name “Nummer” (number) and in the field are only a number linke 036 , 037, 057 and so on. But the Query doesnt work. The gallerie is sort by ID not by the costum field. Here are the builded query : album__in=2&order=ASC&orderby=meta_value

So what can i do? is this an issue?

Regards Marcus