diff --git a/src/taskgraph/morph.py b/src/taskgraph/morph.py
index 357741f24..0e37df020 100644
--- a/src/taskgraph/morph.py
+++ b/src/taskgraph/morph.py
@@ -147,7 +147,9 @@ def make_index_task(parent_task, taskgraph, label_to_taskid, parameters, graph_c
     task.task["payload"]["command"] = ["insert-indexes.js"] + index_paths
     task.task["payload"]["env"] = {
         "TARGET_TASKID": parent_task.task_id,
-        "INDEX_RANK": parent_task.task.get("extra", {}).get("index", {}).get("rank", 0),
+        "INDEX_RANK": str(
+            parent_task.task.get("extra", {}).get("index", {}).get("rank", 0)
+        ),
     }
     return task, taskgraph, label_to_taskid
 
diff --git a/test/test_morph.py b/test/test_morph.py
index a894d14ea..c39150441 100644
--- a/test/test_morph.py
+++ b/test/test_morph.py
@@ -66,7 +66,7 @@ def test_make_index_tasks(make_taskgraph, graph_config):
 
     assert index_task.task["payload"]["command"][0] == "insert-indexes.js"
     assert index_task.task["payload"]["env"]["TARGET_TASKID"] == "a-tid"
-    assert index_task.task["payload"]["env"]["INDEX_RANK"] == 1540722354
+    assert index_task.task["payload"]["env"]["INDEX_RANK"] == "1540722354"
 
     # check the scope summary
     assert index_task.task["scopes"] == ["index:insert-task:gecko.v2.mozilla-central.*"]