diff --git a/unsupported/Eigen/src/MatrixFunctions/MatrixPowerBase.h b/unsupported/Eigen/src/MatrixFunctions/MatrixPowerBase.h
--- a/unsupported/Eigen/src/MatrixFunctions/MatrixPowerBase.h
+++ b/unsupported/Eigen/src/MatrixFunctions/MatrixPowerBase.h
@@ -301,6 +301,7 @@
       res(0,0) = std::pow(m_A(0,0), p);
       break;
     case 2:
+      res.resize(2, 2);
       compute2x2(res, p);
       break;
     default:
@@ -320,6 +321,7 @@
   res += MatrixType::Identity(IminusT.rows(), IminusT.cols());
 }
 
+// this function assumes that res has the correct size (see bug 614)
 template<typename MatrixType>
 void MatrixPowerTriangularAtomic<MatrixType>::compute2x2(MatrixType& res, RealScalar p) const
 {
diff --git a/unsupported/test/matrix_power.cpp b/unsupported/test/matrix_power.cpp
--- a/unsupported/test/matrix_power.cpp
+++ b/unsupported/test/matrix_power.cpp
@@ -181,6 +181,6 @@
   CALL_SUBTEST_1(testMatrixVector(Matrix2f(),         Vector2f(),    1e-4));
   CALL_SUBTEST_5(testMatrixVector(Matrix3cf(),        Vector3cf(),   1e-4));
   CALL_SUBTEST_8(testMatrixVector(Matrix4f(),         Vector4f(),    1e-4));
-  CALL_SUBTEST_6(testMatrixVector(MatrixXf(8,8),      VectorXf(8),   1e-3));
+  CALL_SUBTEST_6(testMatrixVector(MatrixXf(2,2),      VectorXf(2),   1e-3)); // see bug 614
   CALL_SUBTEST_9(testMatrixVector(MatrixXe(7,7),      VectorXe(7),   1e-13));
 }