diff --git a/troposphere/ecs.py b/troposphere/ecs.py
index 0035664d3..5677e24c7 100644
--- a/troposphere/ecs.py
+++ b/troposphere/ecs.py
@@ -35,7 +35,7 @@ class Service(AWSObject):
         'DesiredCount': (positive_integer, False),
         'LoadBalancers': ([LoadBalancer], False),
         'Role': (basestring, False),
-        'TaskDefinition': (basestring, False),
+        'TaskDefinition': (basestring, True),
     }