diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index d9d8458..c28d233 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -519,7 +519,7 @@
         assert group != self._ffi.NULL
 
         nid = self._lib.EC_GROUP_get_curve_name(group)
-        assert nid != 0
+        assert nid != self._lib.NID_undef
 
         curve_name = self._lib.OBJ_nid2sn(nid)
         assert curve_name != self._ffi.NULL