diff --git a/zipline/finance/performance.py b/zipline/finance/performance.py
index 9f32ae09..e237ec7b 100644
--- a/zipline/finance/performance.py
+++ b/zipline/finance/performance.py
@@ -68,7 +68,10 @@ Performance Period
 ==================
 
 Performance Periods are updated with every trade. When calling
-code needs
+code needs a portfolio object that fulfills the algorithm
+protocol, use the PerformancePeriod.as_portfolio method. See that
+method for comments on the specific fields provided (and
+omitted).
 
     +---------------+------------------------------------------------------+
     | key           | value                                                |