diff --git a/include/openssl/x509.h b/include/openssl/x509.h
index 6fb07aa..b3a988b 100644
--- a/include/openssl/x509.h
+++ b/include/openssl/x509.h
@@ -1630,6 +1630,10 @@
 // determined by the extension type. This library represents extensions with the
 // |X509_EXTENSION| type.
 
+// X509_EXTENSION is an |ASN1_ITEM| whose ASN.1 type is X.509 Extension (RFC
+// 5280) and C type is |X509_EXTENSION*|.
+DECLARE_ASN1_ITEM(X509_EXTENSION)
+
 // X509_EXTENSION_new returns a newly-allocated, empty |X509_EXTENSION| object
 // or NULL on error.
 OPENSSL_EXPORT X509_EXTENSION *X509_EXTENSION_new(void);