diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index 2262776ab2..962cc270bc 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -30,6 +30,12 @@ import threading
 
 logger = logging.getLogger(__name__)
 
+try:
+    MAX_TXN_ID = sys.maxint - 1
+except AttributeError:
+    # python 3 does not have a maximum int value
+    MAX_TXN_ID = 2**63 - 1
+
 sql_logger = logging.getLogger("synapse.storage.SQL")
 transaction_logger = logging.getLogger("synapse.storage.txn")
 perf_logger = logging.getLogger("synapse.storage.TIME")
@@ -222,7 +228,7 @@ class SQLBaseStore(object):
 
         # We don't really need these to be unique, so lets stop it from
         # growing really large.
-        self._TXN_ID = (self._TXN_ID + 1) % (sys.maxint - 1)
+        self._TXN_ID = (self._TXN_ID + 1) % (MAX_TXN_ID)
 
         name = "%s-%x" % (desc, txn_id, )