diff --git a/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.cpp b/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.cpp
index aa5670c95..b230288fe 100644
--- a/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.cpp
+++ b/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.cpp
@@ -679,26 +679,27 @@ namespace Opm {
     }
 
     const std::vector<int>& EclipseGrid::getActiveMap() const {
-        if (!activeMap) {
-            std::vector<int> * data_ptr = new std::vector<int>( getNumActive() );
-            std::vector<int>& data = *data_ptr;
-            activeMap.reset( data_ptr );
+        if( !this->activeMap.empty() ) return this->activeMap;
 
-            for (int global_index = 0; global_index < static_cast<int>(getCartesianSize()); global_index++) {
-                // Using the low level C function to get the active index, because the C++
-                // version will throw for inactive cells.
-                int active_index = ecl_grid_get_active_index1( m_grid.get() , global_index );
-                if (active_index >= 0)
-                    data[active_index] = global_index;
-            }
+        this->activeMap.resize( this->getNumActive() );
+        const auto size = int(this->getCartesianSize());
+
+        for( int global_index = 0; global_index < size; global_index++) {
+            // Using the low level C function to get the active index, because the C++
+            // version will throw for inactive cells.
+            int active_index = ecl_grid_get_active_index1( m_grid.get() , global_index );
+            if (active_index >= 0)
+                this->activeMap[ active_index ] = global_index;
         }
 
-        return *activeMap;
+        return this->activeMap;
     }
 
     void EclipseGrid::resetACTNUM( const int * actnum) {
-        activeMap.reset( 0 );
         ecl_grid_reset_actnum( m_grid.get() , actnum );
+        /* re-build the active map cache */
+        this->activeMap.clear();
+        this->getActiveMap();
     }
 
 
diff --git a/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.hpp b/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.hpp
index 371da4c51..9369073f5 100644
--- a/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.hpp
+++ b/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.hpp
@@ -87,9 +87,7 @@ namespace Opm {
         double getMinpvValue( ) const;
 
         /// Will return a vector a length num_active; where the value
-        /// of each element is the corresponding global index. Observe
-        /// that this reference is *invalidated* if there is a call to
-        /// resetACTNUM().
+        /// of each element is the corresponding global index.
         const std::vector<int>& getActiveMap() const;
         std::array<double, 3> getCellCenter(size_t i,size_t j, size_t k) const;
         std::array<double, 3> getCellCenter(size_t globalIndex) const;
@@ -123,7 +121,7 @@ namespace Opm {
         Value<double> m_pinch;
         PinchMode::ModeEnum m_pinchoutMode;
         PinchMode::ModeEnum m_multzMode;
-        mutable std::unique_ptr< std::vector<int> > activeMap;
+        mutable std::vector< int > activeMap;
 
         void initCornerPointGrid(const std::array<int,3>& dims ,
                                  const std::vector<double>& coord ,