diff --git a/require-ess/src/require.c b/require-ess/src/require.c
index 7686f5d84af5eed31d85d982d2c075a7d92759e6..2b4211049240090a2aacb7b22debf5cedd844f9e 100644
--- a/require-ess/src/require.c
+++ b/require-ess/src/require.c
@@ -34,6 +34,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <stddef.h>
 #include <sys/stat.h>
 /* This prototype is missing in older EPICS versions */
 DBCORE_API int epicsStdCall iocshCmd(const char *cmd);