diff --git a/lib/hx509/crypto.c b/lib/hx509/crypto.c
index 8c654ea3d..c914bc781 100644
--- a/lib/hx509/crypto.c
+++ b/lib/hx509/crypto.c
@@ -765,19 +765,21 @@ _hx509_parse_private_key(const heim_oid *key_oid,
  *
  */
 
+static const heim_octet_string null_entry_oid = { 2, "\x05\x00" };
+
 static const unsigned sha1_oid_tree[] = { 1, 3, 14, 3, 2, 26 };
 const AlgorithmIdentifier _hx509_signature_sha1_data = { 
-    { 6, rk_UNCONST(sha1_oid_tree) }, NULL
+    { 6, rk_UNCONST(sha1_oid_tree) }, &null_entry_oid
 };
 
 static const unsigned md5_oid_tree[] = { 1, 2, 840, 113549, 2, 5 };
 const AlgorithmIdentifier _hx509_signature_md5_data = { 
-    { 6, rk_UNCONST(md5_oid_tree) }, NULL
+    { 6, rk_UNCONST(md5_oid_tree) }, &null_entry_oid
 };
 
 static const unsigned md2_oid_tree[] = { 1, 2, 840, 113549, 2, 2 };
 const AlgorithmIdentifier _hx509_signature_md2_data = { 
-    { 6, rk_UNCONST(md2_oid_tree) }, NULL
+    { 6, rk_UNCONST(md2_oid_tree) }, &null_entry_oid
 };
 
 static const unsigned rsa_with_sha1_oid[] ={ 1, 2, 840, 113549, 1, 1, 5 };