diff --git a/require-ess/src/expr.h b/require-ess/src/expr.h
index 3924c3fdfe949b882b209a0ab907d8869d52f376..c3c5983f48b5446fbe167076ec23db010b7f2646 100644
--- a/require-ess/src/expr.h
+++ b/require-ess/src/expr.h
@@ -1,6 +1,8 @@
 #ifndef expr_h
 #define expr_h
 
+#include <stddef.h>
+
 #ifdef __cplusplus
 extern {
 #endif
diff --git a/require-ess/src/strdup.h b/require-ess/src/strdup.h
index f20c9f9bc969727f68746d7bf3383b088e47f391..a3738108a9605b72742ac83bd3a1ad48424951c8 100644
--- a/require-ess/src/strdup.h
+++ b/require-ess/src/strdup.h
@@ -1,5 +1,8 @@
 #ifndef strdup_h
 #define strdup_h
+
+#include <stddef.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif