diff --git a/include/core/trajectory/trajectory_base.h b/include/core/trajectory/trajectory_base.h
index 0645be841b4dd9c68cc2a6f557146101f199b26e..60f81c58afcb881fe8be35aae225c2ee55ac2c69 100644
--- a/include/core/trajectory/trajectory_base.h
+++ b/include/core/trajectory/trajectory_base.h
@@ -53,7 +53,7 @@ class TrajectoryBase : public NodeBase, public std::enable_shared_from_this<Traj
 
     public:
         // factors
-        void getFactorList(FactorBasePtrList & _fac_list);
+        void getFactorList(FactorBasePtrList & _fac_list) const;
 
     protected:
         FrameBaseConstIter computeFrameOrder(FrameBasePtr _frame_ptr);
diff --git a/src/trajectory/trajectory_base.cpp b/src/trajectory/trajectory_base.cpp
index 35a1f9997e7ab5bffa8f4dbf4f8ac7c44415b66b..10bb5b1ec1b6c7d3a9f800b7e5f443d8b6fed0ac 100644
--- a/src/trajectory/trajectory_base.cpp
+++ b/src/trajectory/trajectory_base.cpp
@@ -44,7 +44,7 @@ void TrajectoryBase::removeFrame(FrameBasePtr _frame_ptr)
         updateLastFrames();
 }
 
-void TrajectoryBase::getFactorList(FactorBasePtrList & _fac_list)
+void TrajectoryBase::getFactorList(FactorBasePtrList & _fac_list) const
 {
 	for(auto fr_ptr : getFrameList())
 		fr_ptr->getFactorList(_fac_list);