diff --git a/bigchaindb/core.py b/bigchaindb/core.py
index af265d09..95cccfc7 100644
--- a/bigchaindb/core.py
+++ b/bigchaindb/core.py
@@ -1,6 +1,7 @@
 import random
 import math
 import collections
+from copy import deepcopy
 
 from itertools import compress
 import rethinkdb as r
@@ -130,6 +131,9 @@ class Bigchain(object):
             # I am the only node
             assignee = self.me
 
+        # We copy the transaction here to not add `assignee` to the transaction
+        # dictionary passed to this method (as it would update by reference).
+        signed_transaction = deepcopy(signed_transaction)
         # update the transaction
         signed_transaction.update({'assignee': assignee})
 
@@ -151,7 +155,7 @@ class Bigchain(object):
         Returns:
             A dict with the transaction details if the transaction was found.
             Will add the transaction status to payload ('valid', 'undecided',
-            or 'backlog'). If no transaction with that `txid` was found it 
+            or 'backlog'). If no transaction with that `txid` was found it
             returns `None`
         """
 
diff --git a/tests/test_core.py b/tests/test_core.py
index 91dc9ef5..2650ff37 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -95,3 +95,14 @@ def test_transaction_exists(monkeypatch, items, exists):
         RqlQuery, 'run', lambda x, y: namedtuple('response', 'items')(items))
     bigchain = Bigchain(public_key='pubkey', private_key='privkey')
     assert bigchain.transaction_exists('txid') is exists
+
+
+def test_write_transaction_no_sideffects(b):
+    from rethinkdb.errors import ReqlOpFailedError
+    transaction = {'id': 'abc'}
+    expected = {'id': 'abc'}
+    with pytest.raises(ReqlOpFailedError):
+        b.write_transaction(transaction)
+    assert transaction == expected
+    with pytest.raises(KeyError):
+        transaction['assignee']