diff --git a/zipline/finance/commission.py b/zipline/finance/commission.py
index b7737f05..58a3df3e 100644
--- a/zipline/finance/commission.py
+++ b/zipline/finance/commission.py
@@ -26,7 +26,7 @@ class PerShare(object):
         means three cents per share, which is a very conservative
         (quite high) for per share costs.
         """
-        self.cost = cost
+        self.cost = float(cost)
 
     def calculate(self, transaction):
         """