diff --git a/zipline/finance/performance/period.py b/zipline/finance/performance/period.py
index f027920f..83c0671a 100644
--- a/zipline/finance/performance/period.py
+++ b/zipline/finance/performance/period.py
@@ -351,7 +351,7 @@ class PerformancePeriod(object):
 
     def calculate_positions_value(self):
         if len(self.position_values) == 0:
-            return 0
+            return np.float64(0)
 
         return sum(self.position_values)