diff --git a/zipline/finance/slippage.py b/zipline/finance/slippage.py
index 4b2241ef..7b0f164c 100644
--- a/zipline/finance/slippage.py
+++ b/zipline/finance/slippage.py
@@ -36,8 +36,8 @@ def transact_stub(slippage, commission, event, open_orders):
     with Processor(inject_algo_dt).threadbound():
 
         transaction = slippage.simulate(event, open_orders)
-        if transaction and np.allclose(transaction.amount, 0):
-            direction = abs(transaction.amount) / transaction.amount
+        if transaction and not np.allclose(transaction.amount, 0):
+            direction = math.copysign(1, transaction.amount)
             per_share, total_commission = commission.calculate(transaction)
             transaction.price = transaction.price + (per_share * direction)
             transaction.commission = total_commission