diff --git a/kernel/hashlib.h b/kernel/hashlib.h
index 2da53dd8a..3c824b8c3 100644
--- a/kernel/hashlib.h
+++ b/kernel/hashlib.h
@@ -10,6 +10,7 @@
 // -------------------------------------------------------
 
 #ifndef HASHLIB_H
+#define HASHLIB_H
 
 #include <stdexcept>
 #include <algorithm>