rMBP 15inch Wifi Issue

I've just bought a new rMBP 15 inch a few days ago and for some reason every time it goes into sleep / powernap mode, when I turn it back on the wifi doesn't work. The odd part is the wifi still appears to be connected but I can't transfer any data, go online etc. I have to turn wifi off and on in order to get back online.

Any ideas what's going on? I have Mavericks installed and completely up to date.

I have exactly the same issue on my rMBP, but only with my home router. Every other router works fine, so I've put it down to the router being strange (it's a proprietary ISP router) as opposed to the Mac.