diff --git a/src/robinsonizer_mode.hpp b/src/robinsonizer_mode.hpp
index 6adf643..e4b1722 100644
--- a/src/robinsonizer_mode.hpp
+++ b/src/robinsonizer_mode.hpp
@@ -6,8 +6,13 @@
 // Copyright (c) 2019 SwiftCoder. All rights reserved.
 //
 
+#ifndef _ROBINSONIZER_MODE_H_
+#define _ROBINSONIZER_MODE_H_
+
 enum class robinsonizer_mode {
     pangram,
     lax_autogram,
     strict_autogram
 };
+
+#endif