diff --git a/zipline/finance/performance/period.py b/zipline/finance/performance/period.py
index c292b191..67dec7a7 100644
--- a/zipline/finance/performance/period.py
+++ b/zipline/finance/performance/period.py
@@ -198,9 +198,9 @@ class PerformancePeriod(object):
     def handle_cash_payment(self, payment_amount):
         self.adjust_cash(payment_amount)
 
-    def handle_commission(self, commission):
+    def handle_commission(self, cost):
         # Deduct from our total cash pool.
-        self.adjust_cash(-commission.cost)
+        self.adjust_cash(-cost)
 
     def adjust_cash(self, amount):
         self.period_cash_flow += amount
diff --git a/zipline/finance/performance/position.py b/zipline/finance/performance/position.py
index 9a28ba86..2daf0039 100644
--- a/zipline/finance/performance/position.py
+++ b/zipline/finance/performance/position.py
@@ -165,7 +165,7 @@ class Position(object):
 
         self.amount = total_shares
 
-    def adjust_commission_cost_basis(self, commission):
+    def adjust_commission_cost_basis(self, sid, cost):
         """
         A note about cost-basis in zipline: all positions are considered
         to share a cost basis, even if they were executed in different
@@ -176,9 +176,9 @@ class Position(object):
         all shares in a position.
         """
 
-        if commission.sid != self.sid:
+        if sid != self.sid:
             raise Exception('Updating a commission for a different sid?')
-        if commission.cost == 0.0:
+        if cost == 0.0:
             return
 
         # If we no longer hold this position, there is no cost basis to
@@ -187,7 +187,7 @@ class Position(object):
             return
 
         prev_cost = self.cost_basis * self.amount
-        new_cost = prev_cost + commission.cost
+        new_cost = prev_cost + cost
         self.cost_basis = new_cost / self.amount
 
     def __repr__(self):
diff --git a/zipline/finance/performance/position_tracker.py b/zipline/finance/performance/position_tracker.py
index 8c5c3037..6ef069ad 100644
--- a/zipline/finance/performance/position_tracker.py
+++ b/zipline/finance/performance/position_tracker.py
@@ -317,11 +317,10 @@ class PositionTracker(object):
         position.update(txn)
         self._update_asset(sid)
 
-    def handle_commission(self, commission):
+    def handle_commission(self, sid, cost):
         # Adjust the cost basis of the stock if we own it
-        if commission.sid in self.positions:
-            self.positions[commission.sid].\
-                adjust_commission_cost_basis(commission)
+        if sid in self.positions:
+            self.positions[sid].adjust_commission_cost_basis(sid, cost)
 
     def handle_split(self, split):
         if split.sid in self.positions:
diff --git a/zipline/finance/performance/tracker.py b/zipline/finance/performance/tracker.py
index 558b46cc..a6cf6bf0 100644
--- a/zipline/finance/performance/tracker.py
+++ b/zipline/finance/performance/tracker.py
@@ -314,11 +314,13 @@ class PerformanceTracker(object):
         self.cumulative_performance.record_order(event)
         self.todays_performance.record_order(event)
 
-    def process_commission(self, event):
+    def process_commission(self, commission):
+        sid = commission.sid
+        cost = commission.cost
 
-        self.position_tracker.handle_commission(event)
-        self.cumulative_performance.handle_commission(event)
-        self.todays_performance.handle_commission(event)
+        self.position_tracker.handle_commission(sid, cost)
+        self.cumulative_performance.handle_commission(cost)
+        self.todays_performance.handle_commission(cost)
 
     def process_benchmark(self, event):
         if self.sim_params.data_frequency == 'minute' and \