diff --git a/pandas_eval.py b/pandas_eval.py
index d016fda..5e546e2 100644
--- a/pandas_eval.py
+++ b/pandas_eval.py
@@ -91,7 +91,7 @@ def bollinger(data, window_days, f=2):
 	lower = mid - f * stddev
 	result = pd.DataFrame(upper, index=data.index, columns=['upper'])
 	result['lower'] = lower
-	result['mid'] = stddev
+	result['mid'] = mid
 	return result
 
 data = {}