swprintf return incorrect number of characters written when writing to NULL and 0 length

Expected behavior:

int test = 5;
wchat_t* buff = NULL;
size_t strlen = swprintf(buff ,0 , L"%d", test);

In this instance I would expect strlen = 1;

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Actual behavior:

strlen = 18446744073709551615

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Extra Info:
apt-cache policy libc6
libc6:
  Installed: 2.23-0ubuntu5
  Candidate: 2.23-0ubuntu5
  Version table:
 *** 2.23-0ubuntu5 500
        500 http://gb.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     2.23-0ubuntu3 500
        500 http://gb.archive.ubuntu.com/ubuntu xenial/main amd64 Packages

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lsb_release -rd
Description: Ubuntu 16.04.1 LTS
Release: 16.04