diff --git a/ApplicationLibCode/FileInterface/RifActiveCellsReader.cpp b/ApplicationLibCode/FileInterface/RifActiveCellsReader.cpp
index 7aeeb9d5cb..96a52c7690 100644
--- a/ApplicationLibCode/FileInterface/RifActiveCellsReader.cpp
+++ b/ApplicationLibCode/FileInterface/RifActiveCellsReader.cpp
@@ -53,7 +53,8 @@ std::vector<std::vector<int>> RifActiveCellsReader::activeCellsFromActnumKeyword
 ///
 //--------------------------------------------------------------------------------------------------
 std::vector<std::vector<int>> RifActiveCellsReader::activeCellsFromPorvKeyword( const ecl_file_type* ecl_file,
-                                                                                bool                 dualPorosity )
+                                                                                bool                 dualPorosity,
+                                                                                const int            cellCountMainGrid )
 {
     CAF_ASSERT( ecl_file );
 
@@ -64,19 +65,27 @@ std::vector<std::vector<int>> RifActiveCellsReader::activeCellsFromPorvKeyword(
     //
     // See documentation of active cells in top of ecl_grid.cpp
 
+    bool divideCellCountByTwo = dualPorosity;
+
     int porvKeywordCount = ecl_file_get_num_named_kw( ecl_file, PORV_KW );
     for ( size_t gridIdx = 0; gridIdx < static_cast<size_t>( porvKeywordCount ); gridIdx++ )
     {
         std::vector<double> porvValues;
         RifEclipseOutputFileTools::keywordData( ecl_file, PORV_KW, gridIdx, &porvValues );
 
-        std::vector<int> activeCellsOneGrid;
-
         size_t activeCellCount = porvValues.size();
-        if ( dualPorosity )
+
+        // For some cases, the dual porosity flag is not interpreted correctly. Add a fallback by checking the number of
+        // cells in the main grid
+        // https://github.com/OPM/ResInsight/issues/9833
+        if ( ( gridIdx == 0 ) && ( activeCellCount == cellCountMainGrid * 2 ) ) divideCellCountByTwo = true;
+
+        if ( divideCellCountByTwo )
         {
             activeCellCount /= 2;
         }
+
+        std::vector<int> activeCellsOneGrid;
         activeCellsOneGrid.resize( activeCellCount, 0 );
 
         for ( size_t poreValueIndex = 0; poreValueIndex < porvValues.size(); poreValueIndex++ )
@@ -129,10 +138,10 @@ void RifActiveCellsReader::applyActiveCellsToAllGrids( ecl_grid_type*
             currentGrid = ecl_grid_iget_lgr( ecl_main_grid, gridIndex - 1 );
         }
 
-        auto activeCellsForGrid = activeCellsForAllGrids[gridIndex];
+        const auto& activeCellsForGrid = activeCellsForAllGrids[gridIndex];
         CAF_ASSERT( ecl_grid_get_global_size( currentGrid ) == static_cast<int>( activeCellsForGrid.size() ) );
 
-        int* actnum_values = activeCellsForGrid.data();
+        auto actnum_values = activeCellsForGrid.data();
 
         ecl_grid_reset_actnum( currentGrid, actnum_values );
     }
diff --git a/ApplicationLibCode/FileInterface/RifActiveCellsReader.h b/ApplicationLibCode/FileInterface/RifActiveCellsReader.h
index 0eba9452b9..4b116e47cf 100644
--- a/ApplicationLibCode/FileInterface/RifActiveCellsReader.h
+++ b/ApplicationLibCode/FileInterface/RifActiveCellsReader.h
@@ -32,7 +32,8 @@ class RifActiveCellsReader
 public:
     static std::vector<std::vector<int>> activeCellsFromActnumKeyword( const ecl_file_type* ecl_file );
 
-    static std::vector<std::vector<int>> activeCellsFromPorvKeyword( const ecl_file_type* ecl_file, bool dualPorosity );
+    static std::vector<std::vector<int>>
+        activeCellsFromPorvKeyword( const ecl_file_type* ecl_file, bool dualPorosity, const int cellCountMainGrid );
 
     static void applyActiveCellsToAllGrids( ecl_grid_type*                       ecl_main_grid,
                                             const std::vector<std::vector<int>>& activeCellsForAllGrids );
diff --git a/ApplicationLibCode/FileInterface/RifReaderEclipseOutput.cpp b/ApplicationLibCode/FileInterface/RifReaderEclipseOutput.cpp
index edea05b7ab..9d286cc3ad 100644
--- a/ApplicationLibCode/FileInterface/RifReaderEclipseOutput.cpp
+++ b/ApplicationLibCode/FileInterface/RifReaderEclipseOutput.cpp
@@ -840,8 +840,10 @@ bool RifReaderEclipseOutput::readActiveCellInfo()
                 ecl_file_open( RiaStringEncodingTools::toNativeEncoded( initFileName ).data(), ECL_FILE_CLOSE_STREAM );
             if ( ecl_file )
             {
-                bool isDualPorosity = m_eclipseCase->mainGrid()->isDualPorosity();
-                actnumValuesPerGrid = RifActiveCellsReader::activeCellsFromPorvKeyword( ecl_file, isDualPorosity );
+                bool isDualPorosity    = m_eclipseCase->mainGrid()->isDualPorosity();
+                int  cellCountMainGrid = static_cast<int>( m_eclipseCase->mainGrid()->cellCount() );
+                actnumValuesPerGrid =
+                    RifActiveCellsReader::activeCellsFromPorvKeyword( ecl_file, isDualPorosity, cellCountMainGrid );
                 ecl_file_close( ecl_file );
             }
         }
@@ -2330,8 +2332,10 @@ ecl_grid_type* RifReaderEclipseOutput::loadAllGrids() const
         // TODO : ecl_grid_alloc() will automatically read ACTNUM from EGRID file, and reading of active cell
         // information can be skipped if PORV is available
 
-        bool isDualPorosity = ecl_grid_dual_grid( mainEclGrid );
-        auto activeCells    = RifActiveCellsReader::activeCellsFromPorvKeyword( m_ecl_init_file, isDualPorosity );
+        bool isDualPorosity    = ecl_grid_dual_grid( mainEclGrid );
+        auto cellCountMainGrid = ecl_grid_get_global_size( mainEclGrid );
+        auto activeCells =
+            RifActiveCellsReader::activeCellsFromPorvKeyword( m_ecl_init_file, isDualPorosity, cellCountMainGrid );
 
         if ( !activeCells.empty() )
         {
diff --git a/ApplicationLibCode/UnitTests/RifActiveCellsReader-Test.cpp b/ApplicationLibCode/UnitTests/RifActiveCellsReader-Test.cpp
index dcfddb0dce..2052d8a29a 100644
--- a/ApplicationLibCode/UnitTests/RifActiveCellsReader-Test.cpp
+++ b/ApplicationLibCode/UnitTests/RifActiveCellsReader-Test.cpp
@@ -65,7 +65,8 @@ TEST( RifActiveCellsReaderTest, BasicTest10k )
         ecl_file_type* initFile =
             ecl_file_open( RiaStringEncodingTools::toNativeEncoded( filePath ).data(), ECL_FILE_CLOSE_STREAM );
 
-        activeCellsFromPorv = RifActiveCellsReader::activeCellsFromPorvKeyword( initFile, false );
+        int cellCountMainGrid = 0;
+        activeCellsFromPorv   = RifActiveCellsReader::activeCellsFromPorvKeyword( initFile, false, cellCountMainGrid );
         EXPECT_EQ( 2, (int)activeCellsFromPorv.size() );
 
         ecl_file_close( initFile );