diff --git a/Eigen/src/Core/ArrayBase.h b/Eigen/src/Core/ArrayBase.h
index 941e39938..c53ea0a4f 100644
--- a/Eigen/src/Core/ArrayBase.h
+++ b/Eigen/src/Core/ArrayBase.h
@@ -167,6 +167,11 @@ template<typename Derived> class ArrayBase
     explicit ArrayBase(Index);
     ArrayBase(Index,Index);
     template<typename OtherDerived> explicit ArrayBase(const ArrayBase<OtherDerived>&);
+  protected:
+    // mixing arrays and matrices is not legal
+    template<typename OtherDerived> Derived& operator+=(const MatrixBase<OtherDerived>& mat);
+    // mixing arrays and matrices is not legal
+    template<typename OtherDerived> Derived& operator-=(const MatrixBase<OtherDerived>& mat);
 };
 
 /** replaces \c *this by \c *this - \a other.
diff --git a/Eigen/src/Core/MatrixBase.h b/Eigen/src/Core/MatrixBase.h
index 0c2bbf5bf..1b0784c2b 100644
--- a/Eigen/src/Core/MatrixBase.h
+++ b/Eigen/src/Core/MatrixBase.h
@@ -427,6 +427,11 @@ template<typename Derived> class MatrixBase
     explicit MatrixBase(int);
     MatrixBase(int,int);
     template<typename OtherDerived> explicit MatrixBase(const MatrixBase<OtherDerived>&);
+  protected:
+    // mixing arrays and matrices is not legal
+    template<typename OtherDerived> Derived& operator+=(const ArrayBase<OtherDerived>& array);
+    // mixing arrays and matrices is not legal
+    template<typename OtherDerived> Derived& operator-=(const ArrayBase<OtherDerived>& array);
 };
 
 #endif // EIGEN_MATRIXBASE_H