diff --git a/src/multibody/fwd.hpp b/src/multibody/fwd.hpp
index 24aa801cd7a43b46c2e5cff4e9087f19612e2b53..66604d80c7813e3f6c4b341b022a2bb5515084f7 100644
--- a/src/multibody/fwd.hpp
+++ b/src/multibody/fwd.hpp
@@ -18,6 +18,8 @@
 #ifndef __se3_multibody_fwd_hpp__
 #define __se3_multibody_fwd_hpp__
 
+# include <cstddef> // std::size_t
+
 namespace se3
 {
   typedef std::size_t Index;