diff --git a/cryptography/hazmat/bindings/openssl/backend.py b/cryptography/hazmat/bindings/openssl/backend.py
index 1b19dda..6ab4dc2 100644
--- a/cryptography/hazmat/bindings/openssl/backend.py
+++ b/cryptography/hazmat/bindings/openssl/backend.py
@@ -289,12 +289,18 @@
             )
             assert res != 0
             if operation == self._DECRYPT:
-                assert mode.tag is not None
+                if not mode.tag:
+                    raise ValueError("Authentication tag must be supplied "
+                                     "when decrypting")
                 res = self._backend.lib.EVP_CIPHER_CTX_ctrl(
                     ctx, self._backend.lib.Cryptography_EVP_CTRL_GCM_SET_TAG,
                     len(mode.tag), mode.tag
                 )
                 assert res != 0
+            else:
+                if mode.tag:
+                    raise ValueError("Authentication tag must be None when "
+                                     "encrypting")
 
         # pass key/iv
         res = self._backend.lib.EVP_CipherInit_ex(ctx, self._backend.ffi.NULL,
diff --git a/tests/hazmat/primitives/test_block.py b/tests/hazmat/primitives/test_block.py
index 2806efd..02de386 100644
--- a/tests/hazmat/primitives/test_block.py
+++ b/tests/hazmat/primitives/test_block.py
@@ -26,7 +26,9 @@
     Cipher, algorithms, modes
 )
 
-from .utils import generate_aead_exception_test
+from .utils import (
+    generate_aead_exception_test, generate_aead_tag_exception_test
+)
 
 
 @utils.register_interface(interfaces.CipherAlgorithm)
@@ -135,3 +137,11 @@
         ),
         skip_message="Does not support AES GCM",
     )
+    test_aead_tag_exceptions = generate_aead_tag_exception_test(
+        algorithms.AES,
+        modes.GCM,
+        only_if=lambda backend: backend.cipher_supported(
+            algorithms.AES("\x00" * 16), modes.GCM("\x00" * 12)
+        ),
+        skip_message="Does not support AES GCM",
+    )
diff --git a/tests/hazmat/primitives/test_utils.py b/tests/hazmat/primitives/test_utils.py
index ebb8b5c..c39364c 100644
--- a/tests/hazmat/primitives/test_utils.py
+++ b/tests/hazmat/primitives/test_utils.py
@@ -3,7 +3,7 @@
 from .utils import (
     base_hash_test, encrypt_test, hash_test, long_string_hash_test,
     base_hmac_test, hmac_test, stream_encryption_test, aead_test,
-    aead_exception_test,
+    aead_exception_test, aead_tag_exception_test,
 )
 
 
@@ -29,7 +29,7 @@
         assert exc_info.value.args[0] == "message!"
 
 
-class TestAEADFinalizeTest(object):
+class TestAEADExceptionTest(object):
     def test_skips_if_only_if_returns_false(self):
         with pytest.raises(pytest.skip.Exception) as exc_info:
             aead_exception_test(
@@ -40,6 +40,17 @@
         assert exc_info.value.args[0] == "message!"
 
 
+class TestAEADTagExceptionTest(object):
+    def test_skips_if_only_if_returns_false(self):
+        with pytest.raises(pytest.skip.Exception) as exc_info:
+            aead_tag_exception_test(
+                None, None, None,
+                only_if=lambda backend: False,
+                skip_message="message!"
+            )
+        assert exc_info.value.args[0] == "message!"
+
+
 class TestHashTest(object):
     def test_skips_if_only_if_returns_false(self):
         with pytest.raises(pytest.skip.Exception) as exc_info:
diff --git a/tests/hazmat/primitives/utils.py b/tests/hazmat/primitives/utils.py
index 9aa3a89..705983a 100644
--- a/tests/hazmat/primitives/utils.py
+++ b/tests/hazmat/primitives/utils.py
@@ -353,3 +353,38 @@
     decryptor.update(b"a" * 16)
     with pytest.raises(AttributeError):
         decryptor.tag
+
+
+def generate_aead_tag_exception_test(cipher_factory, mode_factory,
+                                     only_if, skip_message):
+    def test_aead_tag_exception(self):
+        for backend in _ALL_BACKENDS:
+            yield (
+                aead_tag_exception_test,
+                backend,
+                cipher_factory,
+                mode_factory,
+                only_if,
+                skip_message
+            )
+    return test_aead_tag_exception
+
+
+def aead_tag_exception_test(backend, cipher_factory, mode_factory,
+                            only_if, skip_message):
+    if not only_if(backend):
+        pytest.skip(skip_message)
+    cipher = Cipher(
+        cipher_factory(binascii.unhexlify(b"0" * 32)),
+        mode_factory(binascii.unhexlify(b"0" * 24)),
+        backend
+    )
+    with pytest.raises(ValueError):
+        cipher.decryptor()
+    cipher = Cipher(
+        cipher_factory(binascii.unhexlify(b"0" * 32)),
+        mode_factory(binascii.unhexlify(b"0" * 24), b"0" * 16),
+        backend
+    )
+    with pytest.raises(ValueError):
+        cipher.encryptor()