diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 10497e7..93cd82f 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -34,6 +34,7 @@
 #if defined(MBEDTLS_X509_CRT_WRITE_C)
 
 #include "mbedtls/x509_crt.h"
+#include "mbedtls/x509_internal.h"
 #include "mbedtls/oid.h"
 #include "mbedtls/asn1write.h"
 #include "mbedtls/sha1.h"