diff --git a/dgl/ImageBaseWidgets.hpp b/dgl/ImageBaseWidgets.hpp
index 9dfc9425..72271d8c 100644
--- a/dgl/ImageBaseWidgets.hpp
+++ b/dgl/ImageBaseWidgets.hpp
@@ -105,10 +105,10 @@ public:
     void setRotationAngle(int angle);
 
 protected:
-     void onDisplay() override;
-     bool onMouse(const MouseEvent&) override;
-     bool onMotion(const MotionEvent&) override;
-     bool onScroll(const ScrollEvent&) override;
+    void onDisplay() override;
+    bool onMouse(const MouseEvent&) override;
+    bool onMotion(const MotionEvent&) override;
+    bool onScroll(const ScrollEvent&) override;
 
 private:
     struct PrivateData;
diff --git a/dgl/src/ImageBaseWidgets.cpp b/dgl/src/ImageBaseWidgets.cpp
index ae6fa975..83eae356 100644
--- a/dgl/src/ImageBaseWidgets.cpp
+++ b/dgl/src/ImageBaseWidgets.cpp
@@ -299,6 +299,7 @@ ImageBaseKnob<ImageType>::ImageBaseKnob(Widget* const parentWidget,
       KnobEventHandler(this),
       pData(new PrivateData(image))
 {
+    KnobEventHandler::setCallback(pData);
     setOrientation(orientation);
     setSize(pData->imgLayerWidth, pData->imgLayerHeight);
 }
@@ -309,6 +310,7 @@ ImageBaseKnob<ImageType>::ImageBaseKnob(const ImageBaseKnob<ImageType>& imageKno
       KnobEventHandler(this, imageKnob),
       pData(new PrivateData(imageKnob.pData))
 {
+    KnobEventHandler::setCallback(pData);
     setOrientation(imageKnob.getOrientation());
     setSize(pData->imgLayerWidth, pData->imgLayerHeight);
 }