diff --git a/bigchaindb/core.py b/bigchaindb/core.py
index fae16e9d..4fb84722 100644
--- a/bigchaindb/core.py
+++ b/bigchaindb/core.py
@@ -77,7 +77,7 @@ class Bigchain(object):
     federation = property(lambda self: set(self.nodes_except_me + [self.me]))
     """ Set of federation member public keys """
 
-    def write_transaction(self, signed_transaction, **kwargs):
+    def write_transaction(self, signed_transaction):
         """Write the transaction to bigchain.
 
         When first writing a transaction to the bigchain the transaction will be kept in a backlog until
diff --git a/bigchaindb/tendermint/lib.py b/bigchaindb/tendermint/lib.py
index 66b568e2..61d1834b 100644
--- a/bigchaindb/tendermint/lib.py
+++ b/bigchaindb/tendermint/lib.py
@@ -19,21 +19,21 @@ logger = logging.getLogger(__name__)
 TENDERMINT_HOST = getenv('TENDERMINT_HOST', 'localhost')
 TENDERMINT_PORT = getenv('TENDERMINT_PORT', '46657')
 ENDPOINT = 'http://{}:{}/'.format(TENDERMINT_HOST, TENDERMINT_PORT)
+MODE_LIST = ('broadcast_tx_async',
+             'broadcast_tx_sync',
+             'broadcast_tx_commit')
 
 
 class BigchainDB(Bigchain):
 
     def post_transaction(self, transaction, mode):
         """Submit a valid transaction to the mempool."""
-        mode_list = ('broadcast_tx_async',
-                     'broadcast_tx_sync',
-                     'broadcast_tx_commit')
-        if not mode or mode['mode'] not in mode_list:
+        if not mode or mode not in MODE_LIST:
             raise ValidationError(('Mode must be one of the following {}.')
-                                  .format(', '.join(mode_list)))
+                                  .format(', '.join(MODE_LIST)))
 
         payload = {
-            'method': mode['mode'],
+            'method': mode,
             'jsonrpc': '2.0',
             'params': [encode_transaction(transaction.to_dict())],
             'id': str(uuid4())
@@ -41,7 +41,7 @@ class BigchainDB(Bigchain):
         # TODO: handle connection errors!
         requests.post(ENDPOINT, json=payload)
 
-    def write_transaction(self, transaction, **mode):
+    def write_transaction(self, transaction, mode):
         # This method offers backward compatibility with the Web API.
         """Submit a valid transaction to the mempool."""
         self.post_transaction(transaction, mode)
diff --git a/bigchaindb/web/views/transactions.py b/bigchaindb/web/views/transactions.py
index e53058b4..c746f2ad 100644
--- a/bigchaindb/web/views/transactions.py
+++ b/bigchaindb/web/views/transactions.py
@@ -59,6 +59,7 @@ class TransactionListApi(Resource):
         parser.add_argument('mode', type=parameters.valid_mode,
                             default='broadcast_tx_async')
         args = parser.parse_args()
+        mode = str(args['mode'])
 
         pool = current_app.config['bigchain_pool']
 
@@ -90,7 +91,7 @@ class TransactionListApi(Resource):
                     'Invalid transaction ({}): {}'.format(type(e).__name__, e)
                 )
             else:
-                bigchain.write_transaction(tx_obj, **args)
+                bigchain.write_transaction(tx_obj, mode)
 
         response = jsonify(tx)
         response.status_code = 202
diff --git a/tests/tendermint/test_lib.py b/tests/tendermint/test_lib.py
index 76c7c744..3355683c 100644
--- a/tests/tendermint/test_lib.py
+++ b/tests/tendermint/test_lib.py
@@ -76,7 +76,7 @@ def test_write_and_post_transaction(mock_post, b):
                     .sign([alice.private_key]).to_dict()
 
     tx = b.validate_transaction(tx)
-    b.write_transaction(tx, **{'mode': 'broadcast_tx_async'})
+    b.write_transaction(tx, 'broadcast_tx_async')
 
     assert mock_post.called
     args, kwargs = mock_post.call_args
@@ -87,9 +87,9 @@ def test_write_and_post_transaction(mock_post, b):
 
 @patch('requests.post')
 @pytest.mark.parametrize('mode', [
-    {'mode': 'broadcast_tx_async'},
-    {'mode': 'broadcast_tx_sync'},
-    {'mode': 'broadcast_tx_commit'}
+    'broadcast_tx_async',
+    'broadcast_tx_sync',
+    'broadcast_tx_commit'
 ])
 def test_post_transaction_valid_modes(mock_post, b, mode):
     from bigchaindb.models import Transaction
@@ -100,10 +100,10 @@ def test_post_transaction_valid_modes(mock_post, b, mode):
                             asset=None) \
         .sign([alice.private_key]).to_dict()
     tx = b.validate_transaction(tx)
-    b.write_transaction(tx, **mode)
+    b.write_transaction(tx, mode)
 
     args, kwargs = mock_post.call_args
-    assert mode['mode'] == kwargs['json']['method']
+    assert mode == kwargs['json']['method']
 
 
 def test_post_transaction_invalid_mode(b):
@@ -117,4 +117,4 @@ def test_post_transaction_invalid_mode(b):
         .sign([alice.private_key]).to_dict()
     tx = b.validate_transaction(tx)
     with pytest.raises(ValidationError):
-        b.write_transaction(tx, **{'mode': 'nope'})
+        b.write_transaction(tx, 'nope')