diff --git a/moto/ecs/exceptions.py b/moto/ecs/exceptions.py
index 41f4f0eee..4f01ba258 100644
--- a/moto/ecs/exceptions.py
+++ b/moto/ecs/exceptions.py
@@ -16,7 +16,7 @@ class TaskDefinitionNotFoundException(JsonRESTError):
     def __init__(self):
         super().__init__(
             error_type="ClientException",
-            message="The specified task definition does not exist.",
+            message="Unable to describe task definition.",
         )
 
 
diff --git a/moto/ecs/models.py b/moto/ecs/models.py
index 477066664..6ea027735 100644
--- a/moto/ecs/models.py
+++ b/moto/ecs/models.py
@@ -1414,12 +1414,15 @@ class EC2ContainerServiceBackend(BaseBackend):
         for task in tasks.keys():
             if task.endswith(task_id):
                 container_instance_arn = tasks[task].container_instance_arn
-                container_instance = self.container_instances[cluster.name][
-                    container_instance_arn.split("/")[-1]
-                ]
-                self.update_container_instance_resources(
-                    container_instance, tasks[task].resource_requirements, removing=True
-                )
+                if container_instance_arn:
+                    container_instance = self.container_instances[cluster.name][
+                        container_instance_arn.split("/")[-1]
+                    ]
+                    self.update_container_instance_resources(
+                        container_instance,
+                        tasks[task].resource_requirements,
+                        removing=True,
+                    )
                 tasks[task].last_status = "STOPPED"
                 tasks[task].desired_status = "STOPPED"
                 tasks[task].stopped_reason = reason