diff --git a/src/TSMapEditor/Mutations/Classes/HeightMutations/AlterElevationMutationBase.cs b/src/TSMapEditor/Mutations/Classes/HeightMutations/AlterElevationMutationBase.cs
index d84884fb..3851ceb3 100644
--- a/src/TSMapEditor/Mutations/Classes/HeightMutations/AlterElevationMutationBase.cs
+++ b/src/TSMapEditor/Mutations/Classes/HeightMutations/AlterElevationMutationBase.cs
@@ -33,6 +33,14 @@ protected bool IsCellMorphable(MapTile cell)
             return Map.TheaterInstance.Theater.TileSets[Map.TheaterInstance.GetTileSetId(cell.TileIndex)].Morphable;
         }
 
+        protected void Clear()
+        {
+            cellsToProcess.Clear();
+            processedCellsThisIteration.Clear();
+            totalProcessedCells.Clear();
+            undoData.Clear();
+        }
+
         /// <summary>
         /// Adds a cell's data to the undo data structure.
         /// Does nothing if the cell has already been added to the undo data structure.
diff --git a/src/TSMapEditor/Mutations/Classes/HeightMutations/FlattenGroundMutation.cs b/src/TSMapEditor/Mutations/Classes/HeightMutations/FlattenGroundMutation.cs
index 88f144d6..1311ca2f 100644
--- a/src/TSMapEditor/Mutations/Classes/HeightMutations/FlattenGroundMutation.cs
+++ b/src/TSMapEditor/Mutations/Classes/HeightMutations/FlattenGroundMutation.cs
@@ -29,6 +29,8 @@ public FlattenGroundMutation(IMutationTarget mutationTarget, Point2D originCell,
 
         private void FlattenGround()
         {
+            Clear();
+
             int xSize = BrushSize.Width;
             int ySize = BrushSize.Height;