Looking at std::unique_ptr and its nullptr_t constructor

I am trying to understand why unique_ptr has a nullptr_t constructor

constexpr unique_ptr::unique_ptr( nullptr_t );

I had assumed this was because the normal one argument constructor was explicit and thus would reject the nullptr value:

explicit unique_ptr::unique_ptr( pointer p );

But when I build an example it compiler fine:

namespace ThorsAnvil
{
    template<typename T>
    class SmartPointer
    {
        public:
            SmartPointer()      {}
            explicit SmartPointer(T*){}
    };
}


template<typename T>
using SP    = ThorsAnvil::SmartPointer<T>;
int main()
{

    SP<int>     data1;
    SP<int>     data2(new int);  // fine
    SP<int>     data3(nullptr);  // fine
}

Here is the output:

> g++ --version
Configured with: --prefix=/Applications/http://ift.tt/1d5DwEL --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.0.0
Thread model: posix
> g++ -Wall -Wextra -std=c++11 SP1.cpp

Why does std::unique_ptr need the extra constructor that takes a nullptr_t argument?