diff --git a/docs/development/test-vectors.rst b/docs/development/test-vectors.rst
index 40c4959..6a7a35e 100644
--- a/docs/development/test-vectors.rst
+++ b/docs/development/test-vectors.rst
@@ -118,6 +118,9 @@
 * ``bc_path_length_zero.pem`` - An RSA 2048 bit self-signed
   certificate containing a basic constraints extension with a path length of
   zero.
+* ``unsupported_extension.pem`` - An RSA 2048 bit self-signed certificate
+  containing an unsupported extension type. The OID was encoded as
+  "1.2.3.4" with an ``extnValue`` of "value".
 
 Custom X.509 Request Vectors
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/vectors/cryptography_vectors/x509/custom/unsupported_extension.pem b/vectors/cryptography_vectors/x509/custom/unsupported_extension.pem
new file mode 100644
index 0000000..45e799a
--- /dev/null
+++ b/vectors/cryptography_vectors/x509/custom/unsupported_extension.pem
@@ -0,0 +1,18 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----