diff --git a/headers/enduro2d/utils/strings.hpp b/headers/enduro2d/utils/strings.hpp
index 6949f913..e8ad29ed 100644
--- a/headers/enduro2d/utils/strings.hpp
+++ b/headers/enduro2d/utils/strings.hpp
@@ -4,6 +4,8 @@
  * Copyright (C) 2018 Matvey Cherevko
  ******************************************************************************/
 
+#ifndef E2D_INCLUDE_GUARD_4540A3A22AD942D8A6B0C77A3346E73A
+#define E2D_INCLUDE_GUARD_4540A3A22AD942D8A6B0C77A3346E73A
 #pragma once
 
 #include "_utils.hpp"
@@ -56,3 +58,4 @@ namespace e2d
 }
 
 #include "strings.inl"
+#endif
diff --git a/headers/enduro2d/utils/strings.inl b/headers/enduro2d/utils/strings.inl
index 29e43f38..af9e0cdf 100644
--- a/headers/enduro2d/utils/strings.inl
+++ b/headers/enduro2d/utils/strings.inl
@@ -4,6 +4,8 @@
  * Copyright (C) 2018 Matvey Cherevko
  ******************************************************************************/
 
+#ifndef E2D_INCLUDE_GUARD_5BC5A803A3694674845E9953209E8CBE
+#define E2D_INCLUDE_GUARD_5BC5A803A3694674845E9953209E8CBE
 #pragma once
 
 #include "_utils.hpp"
@@ -390,3 +392,5 @@ namespace e2d { namespace strings
         return str(buffer.data(), buffer.data() + actual_format_size);
     }
 }}
+
+#endif