diff --git a/ApplicationLibCode/ProjectDataModel/RimGridCalculation.cpp b/ApplicationLibCode/ProjectDataModel/RimGridCalculation.cpp
index 4ba2d3810c..2fb877995a 100644
--- a/ApplicationLibCode/ProjectDataModel/RimGridCalculation.cpp
+++ b/ApplicationLibCode/ProjectDataModel/RimGridCalculation.cpp
@@ -635,6 +635,7 @@ std::vector<double> RimGridCalculation::getActiveCellValues( const QString&
     size_t gridIndex = 0;
     auto   resultAccessor =
         RigResultAccessorFactory::createFromResultAddress( eclipseCaseData, gridIndex, porosityModel, timeStepToUse, resAddr );
+    if ( resultAccessor.isNull() ) return {};
 
 #pragma omp parallel for
     for ( int i = 0; i < static_cast<int>( activeReservoirCells.size() ); i++ )
diff --git a/ApplicationLibCode/ReservoirDataModel/ResultAccessors/RigAllGridCellsResultAccessor.cpp b/ApplicationLibCode/ReservoirDataModel/ResultAccessors/RigAllGridCellsResultAccessor.cpp
index 59d71eec21..24488b4bbb 100644
--- a/ApplicationLibCode/ReservoirDataModel/ResultAccessors/RigAllGridCellsResultAccessor.cpp
+++ b/ApplicationLibCode/ReservoirDataModel/ResultAccessors/RigAllGridCellsResultAccessor.cpp
@@ -21,8 +21,6 @@
 
 #include "RigGridBase.h"
 
-#include "cafAssert.h"
-
 #include <cmath>
 
 //--------------------------------------------------------------------------------------------------
@@ -42,9 +40,9 @@ double RigAllGridCellsResultAccessor::cellScalar( size_t gridLocalCellIndex ) co
     if ( m_reservoirResultValues->empty() ) return HUGE_VAL;
 
     size_t reservoirCellIndex = m_grid->reservoirCellIndex( gridLocalCellIndex );
-    CAF_ASSERT( reservoirCellIndex < m_reservoirResultValues->size() );
+    if ( reservoirCellIndex >= m_reservoirResultValues->size() ) return HUGE_VAL;
 
-    return m_reservoirResultValues->at( reservoirCellIndex );
+    return ( *m_reservoirResultValues )[reservoirCellIndex];
 }
 
 //--------------------------------------------------------------------------------------------------
@@ -62,9 +60,11 @@ double RigAllGridCellsResultAccessor::cellScalarGlobIdx( size_t globCellIndex )
 {
     if ( m_reservoirResultValues->empty() ) return HUGE_VAL;
 
-    CAF_ASSERT( globCellIndex < m_reservoirResultValues->size() );
+    // The cell index can come from another case, see RimGridCalculation::getActiveCellValues(). Avoid at(), which throws: an
+    // exception escaping an OpenMP loop calls std::terminate.
+    if ( globCellIndex >= m_reservoirResultValues->size() ) return HUGE_VAL;
 
-    return m_reservoirResultValues->at( globCellIndex );
+    return ( *m_reservoirResultValues )[globCellIndex];
 }
 
 //--------------------------------------------------------------------------------------------------
diff --git a/ApplicationLibCode/UnitTests/CMakeLists.txt b/ApplicationLibCode/UnitTests/CMakeLists.txt
index 6a9e74ea07..8e075b09b5 100644
--- a/ApplicationLibCode/UnitTests/CMakeLists.txt
+++ b/ApplicationLibCode/UnitTests/CMakeLists.txt
@@ -160,6 +160,7 @@ set(SOURCE_UNITTEST_FILES
     ${CMAKE_CURRENT_LIST_DIR}/RicWellPathExportMswGeometryPath-Test.cpp
     ${CMAKE_CURRENT_LIST_DIR}/RicMswBranchBuilder-Test.cpp
     ${CMAKE_CURRENT_LIST_DIR}/RigMswTableData-Test.cpp
+    ${CMAKE_CURRENT_LIST_DIR}/RigAllGridCellsResultAccessor-Test.cpp
 )
 
 if(RESINSIGHT_ENABLE_GRPC)
diff --git a/ApplicationLibCode/UnitTests/RigAllGridCellsResultAccessor-Test.cpp b/ApplicationLibCode/UnitTests/RigAllGridCellsResultAccessor-Test.cpp
new file mode 100644
index 0000000000..e2dc4fc9df
--- /dev/null
+++ b/ApplicationLibCode/UnitTests/RigAllGridCellsResultAccessor-Test.cpp
@@ -0,0 +1,64 @@
+/////////////////////////////////////////////////////////////////////////////////
+//
+//  Copyright (C) 2026 Equinor ASA
+//
+//  ResInsight is free software: you can redistribute it and/or modify
+//  it under the terms of the GNU General Public License as published by
+//  the Free Software Foundation, either version 3 of the License, or
+//  (at your option) any later version.
+//
+//  ResInsight is distributed in the hope that it will be useful, but WITHOUT ANY
+//  WARRANTY; without even the implied warranty of MERCHANTABILITY or
+//  FITNESS FOR A PARTICULAR PURPOSE.
+//
+//  See the GNU General Public License at <http://www.gnu.org/licenses/gpl.html>
+//  for more details.
+//
+/////////////////////////////////////////////////////////////////////////////////
+
+#include "gtest/gtest.h"
+
+#include "ResultAccessors/RigAllGridCellsResultAccessor.h"
+
+#include <cmath>
+#include <vector>
+
+//--------------------------------------------------------------------------------------------------
+/// The grid is only used by cellScalar(), the global index accessors read the result vector directly.
+//--------------------------------------------------------------------------------------------------
+TEST( RigAllGridCellsResultAccessorTest, CellScalarGlobIdxReturnsValueForValidIndex )
+{
+    std::vector<double> values = { 1.0, 2.0, 3.0 };
+
+    RigAllGridCellsResultAccessor accessor( nullptr, &values );
+
+    EXPECT_DOUBLE_EQ( 1.0, accessor.cellScalarGlobIdx( 0 ) );
+    EXPECT_DOUBLE_EQ( 3.0, accessor.cellScalarGlobIdx( 2 ) );
+}
+
+//--------------------------------------------------------------------------------------------------
+/// A cell index beyond the result vector must not throw. The accessor is called from OpenMP loops,
+/// where an escaping exception terminates the application.
+//--------------------------------------------------------------------------------------------------
+TEST( RigAllGridCellsResultAccessorTest, CellScalarGlobIdxIsUndefinedForIndexBeyondResultVector )
+{
+    std::vector<double> values = { 1.0, 2.0, 3.0 };
+
+    RigAllGridCellsResultAccessor accessor( nullptr, &values );
+
+    EXPECT_NO_THROW( accessor.cellScalarGlobIdx( 3 ) );
+    EXPECT_DOUBLE_EQ( HUGE_VAL, accessor.cellScalarGlobIdx( 3 ) );
+    EXPECT_DOUBLE_EQ( HUGE_VAL, accessor.cellScalarGlobIdx( 1000 ) );
+}
+
+//--------------------------------------------------------------------------------------------------
+///
+//--------------------------------------------------------------------------------------------------
+TEST( RigAllGridCellsResultAccessorTest, CellScalarGlobIdxIsUndefinedForEmptyResultVector )
+{
+    std::vector<double> values;
+
+    RigAllGridCellsResultAccessor accessor( nullptr, &values );
+
+    EXPECT_DOUBLE_EQ( HUGE_VAL, accessor.cellScalarGlobIdx( 0 ) );
+}