C++ :: Excel XLL Returns Different Results?

I am using Excel 2013, Visual Studio 2015. I began learning about Excel XLL. I wrote simple function which tests whether the argument is missing. Here's code:

#define DllExport __declspec(dllexport)
static LPWSTR rgFuncs[1][7] = { { L"GetSum", L"BU", L"GetSum" } };
DllExport double WINAPI GetSum(LPXLOPER12 arg)
{
if (arg->xltype == xltypeMissing) return -1;
return arg->xltype;
}

This code works as expected: if I miss argument, it gives me -1, and the type otherwise. But when I change code to this:

DllExport double WINAPI GetSum(LPXLOPER12 arg)
{
return (arg->xltype == xltypeMissing) ? -1 : arg->xltype;
}

then, when I miss argument, it gives me 4294967295. Why?