Function to return a lock guard wrapping a private mutex

I have a simple class with the following declared privately:

std::mutex headMutex;  
std::unique_lock<std::mutex> usingHeadNode(){return std::unique_lock<std::mutex>(headMutex);}

my aim is to provide a simple helper function that returns a unique_lock wrapping the mutex, so that my calling code can be

auto headGuard = usingHeadNode();

to lock the mutex with the RAII technique. However compiling this raises an error:

error: use of deleted function ‘std::lock_guard<_Mutex>::lock_guard(const std::lock_guard<_Mutex>&) [with _Mutex = std::mutex]

... which looks like the constructor for the lock_guard class is missing or something? I haven't deleted anything as far as I'm aware...