diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 6cc281a..e530ae8 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -18,6 +18,7 @@
 #include "mbedtls/x509_crt.h"
 #include "mbedtls/asn1write.h"
 #include "mbedtls/error.h"
+#include "mbedtls/oid.h"
 #include "x509_oid.h"
 #include "mbedtls/platform.h"
 #include "mbedtls/platform_util.h"