diff --git a/include/boost/coroutine/detail/symmetric_coroutine_self.hpp b/include/boost/coroutine/detail/symmetric_coroutine_self.hpp
index 6b923cb..1f3180b 100644
--- a/include/boost/coroutine/detail/symmetric_coroutine_self.hpp
+++ b/include/boost/coroutine/detail/symmetric_coroutine_self.hpp
@@ -82,7 +82,7 @@ public:
     }
 
     template< typename Coro, typename X >
-    symmetric_coroutine_self & operator()( Coro & other, X x)
+    symmetric_coroutine_self & operator()( Coro & other, X & x)
     {
         BOOST_ASSERT( other);
 
@@ -115,6 +115,8 @@ private:
     typedef parameters< R & >                   param_type;
     typedef symmetric_coroutine_impl< R & >     impl_type;
 
+    struct dummy {};
+
     BOOST_MOVABLE_BUT_NOT_COPYABLE( symmetric_coroutine_self)
 
     impl_type   *   impl_;
@@ -160,7 +162,7 @@ public:
     }
 
     template< typename Coro, typename X >
-    symmetric_coroutine_self & operator()( Coro & other, X x)
+    symmetric_coroutine_self & operator()( Coro & other, X & x)
     {
         BOOST_ASSERT( other);
 
@@ -226,7 +228,7 @@ public:
     { std::swap( impl_, other.impl_); }
 
     template< typename Coro, typename X >
-    symmetric_coroutine_self & operator()( Coro & other, X x)
+    symmetric_coroutine_self & operator()( Coro & other, X & x)
     {
         BOOST_ASSERT( other);