diff --git a/library/ctr.h b/library/ctr.h
index a6b84cd..aa48fb9 100644
--- a/library/ctr.h
+++ b/library/ctr.h
@@ -7,6 +7,9 @@
  *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
  */
 
+#ifndef MBEDTLS_CTR_H
+#define MBEDTLS_CTR_H
+
 #include "common.h"
 
 /**
@@ -28,3 +31,5 @@
         }
     }
 }
+
+#endif /* MBEDTLS_CTR_H */