How can I filter by an URL Field with REST Interface

Hi developers,

I want to filter a SharePoint list by a field of type URL.

The properties of each item look like this:

<m:properties><d:Productnumber>0012</d:Productnumber><d:Productname>Some Name</d:Productname><d:URL>http://www.somewhere.com, http://www.somewhere.com</d:URL></m:properties>

If I set my filter on Productnumber or Productname it works fine like this:

[...]/_vti_bin/ListData.svc/Products?$filter=Produktnumber eq '0000'

[...]/_vti_bin/ListData.svc/Products?$filter=substringof('Name',Productname)

But I want to filter by URL. When I use startswith "http" I get 0 result items.

Any ideas?

Best regards,

Björn