diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index e66ace131..000b4b5e4 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -25,6 +25,9 @@ Deprecations:
 Changes:
 ^^^^^^^^
 
+- Fix X509Extension __str__() method for unknown extension types
+  `#1239 <https://github.com/pyca/pyopenssl/pull/1239>`_.
+  
 23.2.0 (2023-05-30)
 -------------------
 
diff --git a/src/OpenSSL/crypto.py b/src/OpenSSL/crypto.py
index 5fddc4c30..26a09532e 100644
--- a/src/OpenSSL/crypto.py
+++ b/src/OpenSSL/crypto.py
@@ -108,6 +108,8 @@
 TYPE_DH: int = _lib.EVP_PKEY_DH
 TYPE_EC: int = _lib.EVP_PKEY_EC
 
+X509V3_EXT_ERROR_UNKNOWN = 1 << 16
+
 
 class Error(Exception):
     """
@@ -890,7 +892,9 @@ def __str__(self) -> str:
             return self._subjectAltNameString()
 
         bio = _new_mem_buf()
-        print_result = _lib.X509V3_EXT_print(bio, self._extension, 0, 0)
+        print_result = _lib.X509V3_EXT_print(
+            bio, self._extension, X509V3_EXT_ERROR_UNKNOWN, 0
+        )
         _openssl_assert(print_result != 0)
 
         return _bio_to_string(bio).decode("utf-8")
diff --git a/tests/test_crypto.py b/tests/test_crypto.py
index 7a5d36e82..9d45f7e5a 100644
--- a/tests/test_crypto.py
+++ b/tests/test_crypto.py
@@ -1632,6 +1632,10 @@ def test_undef_oid(self):
             ).get_short_name()
             == b"UNDEF"
         )
+        assert (
+            str(X509Extension(b"1.2.3.4.5.6.7", False, b"DER:05:00"))
+            == "<Not Supported>"
+        )
 
     def test_add_extensions_wrong_args(self):
         """