diff --git a/include/openvpn-plugin.h.in b/include/openvpn-plugin.h.in
index 5e6f8743..aa71b497 100644
--- a/include/openvpn-plugin.h.in
+++ b/include/openvpn-plugin.h.in
@@ -44,6 +44,7 @@ typedef X509 openvpn_x509_cert_t;
 #endif
 
 #include <stdarg.h>
+#include <stddef.h>
 
 #ifdef __cplusplus
 extern "C" {