diff --git a/Eigen/src/Geometry/Quaternion.h b/Eigen/src/Geometry/Quaternion.h
index 67319d15b..e0dfadd7c 100644
--- a/Eigen/src/Geometry/Quaternion.h
+++ b/Eigen/src/Geometry/Quaternion.h
@@ -211,6 +211,7 @@ public:
 template<typename _Scalar>
 struct ei_traits<Quaternion<_Scalar> >
 {
+  typedef Quaternion<_Scalar> PlainObject;
   typedef _Scalar Scalar;
   typedef Matrix<_Scalar,4,1> Coefficients;
   enum{