diff --git a/tests/extmod/asyncio_iterator_event.py b/tests/extmod/asyncio_iterator_event.py
index 6efa6b8645..f61fefcf05 100644
--- a/tests/extmod/asyncio_iterator_event.py
+++ b/tests/extmod/asyncio_iterator_event.py
@@ -50,7 +50,7 @@ def schedule_watchdog(end_ticks):
 async def test(ai):
     for x in range(3):
         await asyncio.sleep(0.1)
-        ai.fetch_data(f"bar {x}")
+        ai.fetch_data("bar {}".format(x))
 
 
 class AsyncIterable: