Passing move-only function arguments to boost::thread constructor

The following works for std::thread. It prints 10 as output, which is what I desire.

void foo(std::unique_ptr<int> && in) {
  std::cout << *in;
}

int main(){
  auto in = std::make_unique<int>(10);
  std::thread t(foo, std::move(in));
  t.join();
} 

But, similar attempt with Boost 1.72 fails to compile

void foo(std::unique_ptr<int> && in) {
    std::cout << *in;
}

int main(){

    auto in = std::make_unique<int>(10);
    boost::thread t(foo, std::move(in));
    t.join();
} 

Error : note: copy constructor of 'list1<boost::_bi::value<std::unique_ptr<int>>>' is implicitly deleted because base class 'storage1<boost::_bi::value<std::unique_ptr<int>>>' has a deleted copy constructor
template< class A1 > class list1: private storage1< A1 >

I find this surprising, because the documentation for boost::thread states the following :

Thread Constructor with arguments template <class F,class A1,class A2,...> thread(F f,A1 a1,A2 a2,...);

Preconditions: F and each An must be copyable or movable.

Since I am passing a std::unique_ptr as argument, I am meeting the 'movable' criterion. So, I wonder why is boost thread constructing the std::unique_ptr? Shouldn't it move the std::unique_ptr into the thread object, and then move it further into the thread function like the implementation for std::thread does?