GetProcessAffinityMask returns null process affinity

In a very simple test console application, I tried to get process' affinity mask:

#include <cstdlib>
#include <cstdio>
#include <windows.h>

int main()
{
    while (1)
    {
        DWORD dwProcessAffinityMask = 0;
        DWORD dwSystemAffinityMask = 0;

        BOOL res = GetProcessAffinityMask(
            GetCurrentProcess(),
            (PDWORD_PTR)&dwProcessAffinityMask,
            (PDWORD_PTR)&dwSystemAffinityMask);

        printf("%d 0x%X 0x%X\n",
            res,
            dwProcessAffinityMask,
            dwSystemAffinityMask);

        Sleep(1000);
    }

    return 0;
}

Here is the output (64-bit executable, 64-bit system, meaning I do not fall into the WoW64 special case):

1 0x0 0x3
1 0x0 0x3
...

Running on my laptop, which has a 2 cores CPU, the resulting system's mask looks correct. But I don't understand the meaning of the dwProcessAffinityMask value I get here. Just for the sake of it, I also tried to toy around with the Task Manager by changing the process' affinity mask but the output remains the same.

This behavior doesn't seem to be documented.