Multisite media path

Hi there,
I've got a mutlisite set-up, but I'm finding an issue with the upload path for the media.

When I go into the backend and upload an image, it is putting the image at the following URL eg... however that address doesn't actually exist as the url is really:

How can I make the uploads into the media library go to the correct place? I've not changed anything from the standard set-up so not sure why this is happening.

Please help :0)