diff --git a/doit/task.py b/doit/task.py
index 4859e33b..e3a6f7a3 100644
--- a/doit/task.py
+++ b/doit/task.py
@@ -407,7 +407,7 @@ def check_attr(task, attr, value, valid):
         @param valid (list): of valid types/value accepted
         @raises InvalidTask if invalid input
         """
-        if isinstance(value, valid[0]):
+        if isinstance(value, tuple(valid[0])):
             return
         if value in valid[1]:
             return