diff --git a/dangerzone/updater/registry.py b/dangerzone/updater/registry.py
index b988e12..74674ef 100644
--- a/dangerzone/updater/registry.py
+++ b/dangerzone/updater/registry.py
@@ -93,7 +93,7 @@ def get_manifest(image_str: str) -> requests.Response:
     return response
 
 
-def list_manifests(image_str) -> list:
+def list_manifests(image_str: str) -> list:
     return get_manifest(image_str).json().get("manifests")