D800 - Blue sky turns white

Hi, I'm new here and hoping someone can help. I've got a new D800 and, in taking some shots of a prefectly blue sky have noticed that they have turned out white. Why would/could that be?

To explain a bit further; Originally I got this result using Auto1 WB but then switched to sunshine in the hopes that that was what I had got wrong. Using this WB, the first shot I took was f1.8 1/3200s at ISO100 and showed a beautiful blue sky. The second shot was at f16, 1/40s at ISO3200 and showed the sky perfectly white.

I'm not a complete novice but nor am I anywhere close to an expert so it could well be something quite basic that I'm not understanding. Thanks for your help.