diff --git a/slack/im.py b/slack/im.py
index 39bcb43..d3458ab 100644
--- a/slack/im.py
+++ b/slack/im.py
@@ -30,7 +30,7 @@ class IM(threads.Thread):
 
 
 def history(im, timestamp):
-    ret = api.call("im.history", token = auth.token(), channel = im)
+    ret = api.call("im.history", token = auth.token(), channel = im, oldest = timestamp)
     if ret["ok"] == False:
         return None
     return ret["messages"]