diff --git a/zerver/webhooks/gci/view.py b/zerver/webhooks/gci/view.py
index 1455214ad1..cbf32aaaaf 100644
--- a/zerver/webhooks/gci/view.py
+++ b/zerver/webhooks/gci/view.py
@@ -12,7 +12,7 @@ GCI_MESSAGE_TEMPLATE = u'**{actor}** {action} the task [{task_name}]({task_url})
 GCI_SUBJECT_TEMPLATE = u'{student_name}'
 
 
-def build_instance_url(instance_id):
+def build_instance_url(instance_id: str) -> str:
     return "https://codein.withgoogle.com/dashboard/task-instances/{}/".format(instance_id)
 
 class UnknownEventType(Exception):