diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index d4aaad7..1fcd7ef 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -471,6 +471,8 @@
                     "The length of the provided data is not a multiple of "
                     "the block length."
                 )
+            else:
+                raise self._backend._unknown_error(errors[0])
 
         if (isinstance(self._mode, GCM) and
            self._operation == self._ENCRYPT):
diff --git a/tests/hazmat/backends/test_openssl.py b/tests/hazmat/backends/test_openssl.py
index 9d48f4e..42c1b39 100644
--- a/tests/hazmat/backends/test_openssl.py
+++ b/tests/hazmat/backends/test_openssl.py
@@ -14,7 +14,7 @@
 import pytest
 
 from cryptography import utils
-from cryptography.exceptions import UnsupportedAlgorithm
+from cryptography.exceptions import UnsupportedAlgorithm, InternalError
 from cryptography.hazmat.backends.openssl.backend import backend, Backend
 from cryptography.hazmat.primitives import interfaces, hashes
 from cryptography.hazmat.primitives.ciphers import Cipher
@@ -118,6 +118,15 @@
             b"data not multiple of block length"
         )
 
+    def test_unknown_error_in_cipher_finalize(self):
+        cipher = Cipher(AES(b"\0" * 16), CBC(b"\0" * 16), backend=backend)
+        enc = cipher.encryptor()
+        enc.update(b"\0")
+        backend._lib.ERR_put_error(0, 0, 1,
+                                   b"test_openssl.py", -1)
+        with pytest.raises(InternalError):
+            enc.finalize()
+
     def test_derive_pbkdf2_raises_unsupported_on_old_openssl(self):
         if backend.pbkdf2_hmac_supported(hashes.SHA256()):
             pytest.skip("Requires an older OpenSSL")