How do I define a negative UDL in c++11 (are they disallowed?)?

I'm not even sure if negative User-Defined Literals are allowed. If not, why were they left out?

For example, I would like to use:

auto money_i_owe_jack = -52_jpy;

This is what I tried using gcc 4.7.2:

constexpr int64_t operator "" _jpy(long long l)
{
  return static_cast<int64_t>(l);
}

ERROR

Test_udl.cpp:60:47: error: ‘constexpr int64_t operator"" _jpy(long long int)’ has invalid argument list