diff --git a/Eigen/src/Core/MatrixBase.h b/Eigen/src/Core/MatrixBase.h
index c98da062f..594fbedf7 100644
--- a/Eigen/src/Core/MatrixBase.h
+++ b/Eigen/src/Core/MatrixBase.h
@@ -137,7 +137,7 @@ template<typename Derived> class MatrixBase
     /** \internal the return type of identity */
     typedef CwiseNullaryOp<internal::scalar_identity_op<Scalar>,Derived> IdentityReturnType;
     /** \internal the return type of unit vectors */
-    typedef Block<CwiseNullaryOp<internal::scalar_identity_op<Scalar>, SquareMatrixType>,
+    typedef Block<const CwiseNullaryOp<internal::scalar_identity_op<Scalar>, SquareMatrixType>,
                   internal::traits<Derived>::RowsAtCompileTime,
                   internal::traits<Derived>::ColsAtCompileTime> BasisReturnType;
 #endif // not EIGEN_PARSED_BY_DOXYGEN