diff --git a/src/remove_external_cells_filter.cc b/src/remove_external_cells_filter.cc
index c16f2e0..f3a2455 100644
--- a/src/remove_external_cells_filter.cc
+++ b/src/remove_external_cells_filter.cc
@@ -126,7 +126,7 @@ vtkTypeBool RemoveExternalCellsFilter::RequestData(
 	vtkNew<vtkPoints> outPoints;
 	vtkNew<vtkIdList> outPointIds;
 	vtkIdType outPointId = 0;
-	outPointIds->Allocate(3);
+	outPointIds->SetNumberOfIds(4);
 	output->Allocate();
 	auto it = tetMesh->NewCellIterator();
 	for (it->InitTraversal();
@@ -140,12 +140,14 @@ vtkTypeBool RemoveExternalCellsFilter::RequestData(
 			outPointIds->SetId(0, outPointId + 0);
 			outPointIds->SetId(1, outPointId + 1);
 			outPointIds->SetId(2, outPointId + 2);
-			outPointId += 3;
+			outPointIds->SetId(3, outPointId + 3);
+			outPointId += 4;
 			outPoints->InsertPoints(outPointIds, pointIds,
 									tetMesh->GetPoints());
 			output->InsertNextCell(it->GetCellType(), outPointIds);
 		}
 	}
+	outPoints->SetNumberOfPoints(outPointId + 3);
 	output->SetPoints(outPoints);
 
 	return true;