Eigen/src/StlSupport/details.h |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/Eigen/src/StlSupport/details.h b/Eigen/src/StlSupport/details.h
index 397c8ef..c1ac745 100644
--- a/Eigen/src/StlSupport/details.h
+++ b/Eigen/src/StlSupport/details.h
@@ -30,6 +30,8 @@
   #define EIGEN_ALIGNED_ALLOCATOR Eigen::aligned_allocator
 #endif
 
+#include <cstddef>
+
 namespace Eigen {
 
   // This one is needed to prevent reimplementing the whole std::vector.