diff --git a/library/x509.c b/library/x509.c
index 54275eb..f315821 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -21,6 +21,7 @@
 
 #include "mbedtls/asn1.h"
 #include "mbedtls/error.h"
+#include "mbedtls/oid.h"
 #include "x509_oid.h"
 
 #include <stdio.h>