Why can't unique_ptr infer the type of the deleter?

Let's say I want to use a custom deleter with an unique_ptr:

void custom_deleter(int* obj)
{
    delete obj; 
}

Why do I have to write this:

std::unique_ptr<int, void(*)(int*)> x(new int, custom_deleter);

instead of this:

std::unique_ptr<int> x(new int, custom_deleter); //does not compile

?

Can't the type of the deleter be inferred?