diff --git a/lib/libspl/include/sys/string.h b/lib/libspl/include/sys/string.h
index 3b2f59002..55ccbd090 100644
--- a/lib/libspl/include/sys/string.h
+++ b/lib/libspl/include/sys/string.h
@@ -1 +1,4 @@
+#ifndef _LIBSPL_SYS_STRING_H
+#define	_LIBSPL_SYS_STRING_H
 #include <string.h>
+#endif