Why is the checkbox 'Prefer 32-bit' disabled in Visual Studio 2012?

Question

I came across a situation in which I would like to set the Prefer 32-bit to true. But in Visual Studio 2012, it is showing as disabled. And no matter what I do

Answer 1

In case anybody encounters this for a .NET 4.5 executable in Visual Studio 2013 (and possibly Visual Studio 2012):

We solved it by switching the project to .NET 4.0 and then back .NET 4.5 (on the Build tab of the project properties). This reenabled the checkbox "Prefer 32 bit" for AnyCPU.