Expression 'i < 0' is always false

For the following snippet:

size_t i = 0;
std::wstring s;
s = (i < 0)   ? L"ABC" : L"DEF";
s = (i != -1) ? L"ABC" : L"DEF";

PVS-Studio analysis logs warning for the first condition i < 0, as expected:

V547 Expression 'i < 0' is always false. Unsigned type value is never < 0. test_cpp_vs2017.cpp 19

Why PVS does not issue any warning about the second, also suspicious condition i != -1 reporting it as always true, for instance?