diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 44b6b17..2a1a5e2 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -16,6 +16,7 @@
 #if defined(MBEDTLS_X509_CRT_WRITE_C)
 
 #include "mbedtls/x509_crt.h"
+#include "x509_internal.h"
 #include "mbedtls/asn1write.h"
 #include "mbedtls/error.h"
 #include "mbedtls/oid.h"