diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index 059cfd76f9c45b7268649f031b621971f2d505c4_c3JjL2NyeXB0b2dyYXBoeS9oYXptYXQvYmFja2VuZHMvb3BlbnNzbC9iYWNrZW5kLnB5..daa712c50090814fef73f74783585bd9a6842812_c3JjL2NyeXB0b2dyYXBoeS9oYXptYXQvYmFja2VuZHMvb3BlbnNzbC9iYWNrZW5kLnB5 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -743,7 +743,7 @@
         elif not isinstance(private_key, (rsa.RSAPrivateKey, dsa.DSAPrivateKey,
                             ec.EllipticCurvePrivateKey)):
             raise TypeError(
-                "Key must be rsa, dsa, ec, ed25519 or ed448 private key."
+                "Key must be an rsa, dsa, ec, ed25519, or ed448 private key."
             )
         elif not isinstance(algorithm, hashes.HashAlgorithm):
             raise TypeError("Algorithm must be a registered hash algorithm.")