diff --git a/src/doc/cel.h b/src/doc/cel.h
index 1d57e9096..c0aa7a534 100644
--- a/src/doc/cel.h
+++ b/src/doc/cel.h
@@ -43,7 +43,7 @@ namespace doc {
     void setPosition(int x, int y) { m_x = x; m_y = y; }
     void setOpacity(int opacity) { m_opacity = opacity; }
 
-    int getMemSize() const {
+    virtual int getMemSize() const override {
       return sizeof(Cel);
     }
 
diff --git a/src/doc/image.h b/src/doc/image.h
index 91f75a77a..7df301043 100644
--- a/src/doc/image.h
+++ b/src/doc/image.h
@@ -46,7 +46,7 @@ namespace doc {
     color_t maskColor() const { return m_maskColor; }
     void setMaskColor(color_t c) { m_maskColor = c; }
 
-    int getMemSize() const override;
+    virtual int getMemSize() const override;
     int getRowStrideSize() const;
     int getRowStrideSize(int pixels_per_row) const;
 
diff --git a/src/doc/layer.h b/src/doc/layer.h
index e18f2c9d6..37c13fa6c 100644
--- a/src/doc/layer.h
+++ b/src/doc/layer.h
@@ -42,7 +42,7 @@ namespace doc {
   public:
     virtual ~Layer();
 
-    int getMemSize() const;
+    virtual int getMemSize() const override;
 
     std::string name() const { return m_name; }
     void setName(const std::string& name) { m_name = name; }
@@ -90,7 +90,7 @@ namespace doc {
     explicit LayerImage(Sprite* sprite);
     virtual ~LayerImage();
 
-    int getMemSize() const;
+    virtual int getMemSize() const override;
 
     int getBlendMode() const { return BLEND_MODE_NORMAL; }
 
@@ -124,7 +124,7 @@ namespace doc {
     explicit LayerFolder(Sprite* sprite);
     virtual ~LayerFolder();
 
-    int getMemSize() const;
+    virtual int getMemSize() const override;
 
     const LayerList& getLayersList() { return m_layers; }
     LayerIterator getLayerBegin() { return m_layers.begin(); }
diff --git a/src/doc/mask.h b/src/doc/mask.h
index f302401ef..b94b33631 100644
--- a/src/doc/mask.h
+++ b/src/doc/mask.h
@@ -25,7 +25,7 @@ namespace doc {
     Mask(int x, int y, Image* bitmap);
     virtual ~Mask();
 
-    int getMemSize() const;
+    virtual int getMemSize() const override;
 
     void setName(const char *name);
     const std::string& name() const { return m_name; }
diff --git a/src/doc/sprite.h b/src/doc/sprite.h
index b6796997e..d260ebb83 100644
--- a/src/doc/sprite.h
+++ b/src/doc/sprite.h
@@ -65,7 +65,7 @@ namespace doc {
     color_t transparentColor() const { return m_transparentColor; }
     void setTransparentColor(color_t color);
 
-    int getMemSize() const;
+    virtual int getMemSize() const override;
 
     ////////////////////////////////////////
     // Layers