diff --git a/Sources/secp256k1/scalar_4x64_impl.h b/Sources/secp256k1/scalar_4x64_impl.h
index 685bc99e6..a4529bad0 100755
--- a/Sources/secp256k1/scalar_4x64_impl.h
+++ b/Sources/secp256k1/scalar_4x64_impl.h
@@ -3,6 +3,7 @@
  * Distributed under the MIT software license, see the accompanying   *
  * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
  **********************************************************************/
+#include <sys/types.h>
 
 #ifndef SECP256K1_SCALAR_REPR_IMPL_H
 #define SECP256K1_SCALAR_REPR_IMPL_H