diff --git a/Eigen/Core b/Eigen/Core
index e5bf2f207..9ad5cfabb 100644
--- a/Eigen/Core
+++ b/Eigen/Core
@@ -52,6 +52,7 @@
   #endif
 #endif
 
+#include <cstddef>
 #include <cstdlib>
 #include <cmath>
 #include <complex>