Approximate Time policy compiles for two topics of same type, but fails for topics of different types

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Hello, I am trying to synchronize the callbacks for two topics, one of them, a geometry_msgs::PoseStamped message that acts as a signal for image capture from another computer (just using PoseStamped as a test case for now because it comes with a timestamp, will write own message type later on), and then the actual image topic of type sensor_msgs::Image. This is the code snippet that's doing the approximate time synchronization.

void callback(const geometry_msgs::PoseStamped& signal, const sensor_msgs::ImageConstPtr& rightimage)
{
   ROS_INFO("Reached callback");
   cv_bridge::CvImagePtr imgPtr;
   std::stringstream imgPath;

   //More code
}
typedef sync_policies::ApproximateTime<geometry_msgs::PoseStamped, sensor_msgs::Image> ApproxSyncPol;

Subscriber<geometry_msgs::PoseStamped> signal_sub(nh, "/signal", 1);;
Subscriber<sensor_msgs::Image> image_sub(nh, "camera/image_mono", 1);

Synchronizer<ApproxSyncPol> syncPol(ApproxSyncPol(10), signal_sub, image_sub);

syncPol.registerCallback(boost::bind(&callback, _1, _2));

This code fails to compile with a lot of errors, mostly relating to boost and the Approximate Time sync_policies statement. An excerpt:

 /opt/ros/indigo/include/message_filters/signal9.h:273:23:   required from ‘message_filters::Connection message_filters::Signal9<M0, M1, M2, M3, M4, M5, M6, M7, M8>::addCallback(C&) [with C = const boost::_bi::bind_t<void, void (*)(const geometry_msgs::PoseStamped_<std::allocator<void> >&, const boost::shared_ptr<const sensor_msgs::Image_<std::allocator<void> > >&), boost::_bi::list2<boost::arg<1>, boost::arg<2> > >; M0 = geometry_msgs::PoseStamped_<std::allocator<void> >; M1 = sensor_msgs::Image_<std::allocator<void> >; M2 = message_filters::NullType; M3 = message_filters::NullType; M4 = message_filters::NullType; M5 = message_filters::NullType; M6 = message_filters::NullType; M7 = message_filters::NullType; M8 = message_filters::NullType]’

 /opt/ros/indigo/include/message_filters/synchronizer.h:310:40:   required from ‘message_filters::Connection message_filters::Synchronizer<Policy>::registerCallback(const C&) [with C = boost::_bi::bind_t<void, void (*)(const geometry_msgs::PoseStamped_<std::allocator<void> >&, const boost::shared_ptr<const sensor_msgs::Image_<std::allocator<void> > >&), boost::_bi::list2<boost::arg<1>, boost::arg<2> > >; Policy = message_filters::sync_policies::ApproximateTime<geometry_msgs::PoseStamped_<std::allocator<void> >, sensor_msgs::Image_<std::allocator<void> > >]’

  /usr/include/boost/bind/bind.hpp:313:34: error: invalid initialization of reference of type ‘const geometry_msgs::PoseStamped_<std::allocator<void> >&’ from expression of type ‘const boost::shared_ptr<const geometry_msgs::PoseStamped_<std::allocator<void> > >’
       unwrapper<F>::unwrap(f, 0)(a[base_type::a1_], a[base_type::a2_]);

But this same structure works perfectly, if both topics are sensor_msgs::Image. I am not getting what I'm doing wrong here, because geometry_msgs::PoseStamped is a message that does contain a timestamp.