diff --git a/src/ZMPRefTrajectoryGeneration/problem-vel-ref.hh b/src/ZMPRefTrajectoryGeneration/problem-vel-ref.hh
index 1dfabacfbfecb60e4b89632fc678eb1f3fa97abb..901722fbe3dc2e0b84017430ebb5528d399c4475 100644
--- a/src/ZMPRefTrajectoryGeneration/problem-vel-ref.hh
+++ b/src/ZMPRefTrajectoryGeneration/problem-vel-ref.hh
@@ -27,6 +27,8 @@
 #ifndef _PROBLEM_VEL_REF_H_
 #define _PROBLEM_VEL_REF_H_
 
+#include <iostream>
+
 namespace PatternGeneratorJRL {
 
 /*! \brief Final optimization problem to handle velocity reference.