diff --git a/src/slic3r/GUI/GLGizmo.cpp b/src/slic3r/GUI/GLGizmo.cpp
index 7119fb367..c3587042b 100644
--- a/src/slic3r/GUI/GLGizmo.cpp
+++ b/src/slic3r/GUI/GLGizmo.cpp
@@ -1291,6 +1291,13 @@ std::string GLGizmoFlatten::on_get_name() const
     return L("Flatten");
 }
 
+bool GLGizmoFlatten::on_is_activable(const GLCanvas3D::Selection& selection) const
+{
+    return (selection.is_from_single_object()
+        && (selection.is_single_full_instance() || selection.is_multiple_full_instance() || selection.is_single_full_object())
+        && !selection.is_wipe_tower()           && !selection.is_modifier());
+}
+
 void GLGizmoFlatten::on_start_dragging(const GLCanvas3D::Selection& selection)
 {
     if (m_hover_id != -1)
diff --git a/src/slic3r/GUI/GLGizmo.hpp b/src/slic3r/GUI/GLGizmo.hpp
index e06cfa4bd..4ea049239 100644
--- a/src/slic3r/GUI/GLGizmo.hpp
+++ b/src/slic3r/GUI/GLGizmo.hpp
@@ -435,7 +435,7 @@ public:
 protected:
     virtual bool on_init();
     virtual std::string on_get_name() const;
-    virtual bool on_is_activable(const GLCanvas3D::Selection& selection) const { return (selection.is_from_single_object() && !selection.is_wipe_tower() && !selection.is_modifier());  }
+    virtual bool on_is_activable(const GLCanvas3D::Selection& selection) const;
     virtual void on_start_dragging(const GLCanvas3D::Selection& selection);
     virtual void on_update(const UpdateData& data) {}
     virtual void on_render(const GLCanvas3D::Selection& selection) const;