diff --git a/zipline/finance/risk/risk.py b/zipline/finance/risk/risk.py
index 7d5e1455..5ecc362e 100644
--- a/zipline/finance/risk/risk.py
+++ b/zipline/finance/risk/risk.py
@@ -107,7 +107,7 @@ def sharpe_ratio(algorithm_volatility, algorithm_return, treasury_return):
 def downside_risk(algorithm_returns, mean_returns, normalization_factor):
     rets = algorithm_returns
     mar = mean_returns
-    downside_diff = (rets[rets < mar] - mar).valid()
+    downside_diff = (rets[rets < mar] - mar[rets < mar])
     return np.std(downside_diff) * math.sqrt(normalization_factor)