It's part of the Win64 calling convention. It's normally there to let the callee swap out the arguments from RCX, RDX, R8 and R9, but he may use it for anything. This space must be provided even if the function doesn't take any arguments. It was a bug that this was missing before.

You can see it here:
http://msdn.microsoft.com/en-us/library/ew5tede7.aspx
On that site it isn't named, but in other parts of the documentation, it is called "shadow space".