diff --git a/moto/iot/models.py b/moto/iot/models.py
index 1f0623eb3..2e9979bda 100644
--- a/moto/iot/models.py
+++ b/moto/iot/models.py
@@ -849,7 +849,9 @@ class IoTBackend(BaseBackend):
         ]
         if len(child_groups) > 0:
             raise InvalidRequestException(
-                f" Cannot delete thing group : {thing_group_name} when there are still child groups attached to it"
+                " Cannot delete thing group : "
+                + thing_group_name
+                + " when there are still child groups attached to it"
             )
         thing_group = self.describe_thing_group(thing_group_name)
         del self.thing_groups[thing_group.arn]