diff --git a/Eigen/Core b/Eigen/Core
index 55c8e8195..569062628 100644
--- a/Eigen/Core
+++ b/Eigen/Core
@@ -143,6 +143,7 @@
 #ifdef EIGEN_HAS_ERRNO
 #include <cerrno>
 #endif
+#include <cstddef>
 #include <cstdlib>
 #include <cmath>
 #include <complex>