diff --git a/zipline/finance/trading.py b/zipline/finance/trading.py
index 208dc79c..e6d1fb31 100644
--- a/zipline/finance/trading.py
+++ b/zipline/finance/trading.py
@@ -260,7 +260,7 @@ class OrderDataSource(qmsg.DataSource):
 
 class TransactionSimulator(qmsg.BaseTransform):
     
-    def __init__(self, style): 
+    def __init__(self, style=SIMULATION_STYLE.PARTIAL_VOLUME): 
         qmsg.BaseTransform.__init__(self, zp.TRANSFORM_TYPE.TRANSACTION)
         self.open_orders                = {}
         self.order_count                = 0