diff --git a/zipline/algorithm.py b/zipline/algorithm.py
index 354a7fbe..5307cbee 100644
--- a/zipline/algorithm.py
+++ b/zipline/algorithm.py
@@ -129,7 +129,7 @@ class TradingAlgorithm(object):
             script : str
                 Algoscript that contains initialize and
                 handle_data function definition.
-            data_frequency : str (daily, hourly or minutely)
+            data_frequency : {'daily', 'minute'}
                The duration of the bars.
             capital_base : float <default: 1.0e5>
                How much capital to start with.