diff --git a/moto/ecs/models.py b/moto/ecs/models.py
index 3a2ec13ff..65f44c8bf 100644
--- a/moto/ecs/models.py
+++ b/moto/ecs/models.py
@@ -375,8 +375,8 @@ class EC2ContainerServiceBackend(BaseBackend):
         cluster_service_pair = '{0}:{1}'.format(cluster_name, service_name)
         if cluster_service_pair in self.services:
             if task_definition_str is not None:
-                task_definition = self.fetch_task_definition(task_definition_str)
-                self.services[cluster_service_pair].task_definition = task_definition
+                self.fetch_task_definition(task_definition_str)
+                self.services[cluster_service_pair].task_definition = task_definition_str
             if desired_count is not None:
                 self.services[cluster_service_pair].desired_count = desired_count
             return self.services[cluster_service_pair]
diff --git a/tests/test_ecs/test_ecs_boto3.py b/tests/test_ecs/test_ecs_boto3.py
index 921dea69b..271545efc 100644
--- a/tests/test_ecs/test_ecs_boto3.py
+++ b/tests/test_ecs/test_ecs_boto3.py
@@ -398,6 +398,7 @@ def test_update_service():
     response = client.update_service(
         cluster='test_ecs_cluster',
         service='test_ecs_service',
+        taskDefinition='test_ecs_task',
         desiredCount=0
     )
     response['service']['desiredCount'].should.equal(0)