diff --git a/docs/development/test-vectors.rst b/docs/development/test-vectors.rst
index 15c3be9..81998f7 100644
--- a/docs/development/test-vectors.rst
+++ b/docs/development/test-vectors.rst
@@ -248,6 +248,8 @@
   policy constraints extension with an inhibit policy mapping element.
 * ``pc_require.pem`` - An RSA 2048 bit self-signed certificate containing a
   policy constraints extension with a require explicit policy element.
+* ``unsupported_subject_public_key_info.pem`` - A certificate whose public key
+  is an unknown OID (``1.3.6.1.4.1.8432.1.1.2``).
 
 Custom X.509 Request Vectors
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/src/cryptography/hazmat/backends/openssl/x509.py b/src/cryptography/hazmat/backends/openssl/x509.py
index 80f32e2..eefcb20 100644
--- a/src/cryptography/hazmat/backends/openssl/x509.py
+++ b/src/cryptography/hazmat/backends/openssl/x509.py
@@ -260,7 +260,9 @@
 
     def public_key(self):
         pkey = self._backend._lib.X509_get_pubkey(self._x509)
-        self._backend.openssl_assert(pkey != self._backend._ffi.NULL)
+        if pkey == self._backend._ffi.NULL:
+            raise ValueError("Certificate public key is of an unkonwn type")
+
         pkey = self._backend._ffi.gc(pkey, self._backend._lib.EVP_PKEY_free)
 
         return self._backend._evp_pkey_to_public_key(pkey)
diff --git a/tests/test_x509.py b/tests/test_x509.py
index 0c022df..8035886 100644
--- a/tests/test_x509.py
+++ b/tests/test_x509.py
@@ -2395,6 +2395,21 @@
         ]
 
 
+@pytest.mark.requires_backend_interface(interface=X509Backend)
+class TestOtherCertificate(object):
+    def test_unsupported_subject_public_key_info(self, backend):
+        cert = _load_cert(
+            os.path.join(
+                "x509", "custom", "unsupported_subject_public_key_info.pem"
+            ),
+            x509.load_pem_x509_certificate,
+            backend,
+        )
+
+        with pytest.raises(ValueError):
+            cert.public_key()
+
+
 class TestNameAttribute(object):
     def test_init_bad_oid(self):
         with pytest.raises(TypeError):
diff --git a/vectors/cryptography_vectors/x509/custom/unsupported_subject_public_key_info.pem b/vectors/cryptography_vectors/x509/custom/unsupported_subject_public_key_info.pem
new file mode 100644
index 0000000..aa06dfb
--- /dev/null
+++ b/vectors/cryptography_vectors/x509/custom/unsupported_subject_public_key_info.pem
@@ -0,0 +1,28 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----