Server Add with remote path

I'd like to add the following path to the Server Add module in a Gallery3 setup on a local server:

//192.168.1.220/Documents/My Pictures

This is a share on a local NAS box. I get the "This directory is not readable by the webserver" error. In my Apache config I've set up an alias that points "/myp" to "//192.168.1.220/Documents/My Pictures." If I access "http://localhost/myp" in Firefox then it shows me the contents of "//192.168.1.220/Documents/My Pictures," so as far as Apache is concerned, it is readable.

How can I convince Gallery3 that it is readable?