diff --git a/swh/storage/cassandra/storage.py b/swh/storage/cassandra/storage.py
--- a/swh/storage/cassandra/storage.py
+++ b/swh/storage/cassandra/storage.py
@@ -607,7 +607,7 @@
 
         return {"snapshot:add": len(snapshots)}
 
-    def snapshot_missing(self, snapshots):
+    def snapshot_missing(self, snapshots: List[Sha1Git]) -> Iterable[Sha1Git]:
         return self._cql_runner.snapshot_missing(snapshots)
 
     def snapshot_get(self, snapshot_id):
diff --git a/swh/storage/in_memory.py b/swh/storage/in_memory.py
--- a/swh/storage/in_memory.py
+++ b/swh/storage/in_memory.py
@@ -608,7 +608,7 @@
 
         return {"snapshot:add": count}
 
-    def snapshot_missing(self, snapshots):
+    def snapshot_missing(self, snapshots: List[Sha1Git]) -> Iterable[Sha1Git]:
         for id in snapshots:
             if id not in self._snapshots:
                 yield id
diff --git a/swh/storage/interface.py b/swh/storage/interface.py
--- a/swh/storage/interface.py
+++ b/swh/storage/interface.py
@@ -677,11 +677,11 @@
         ...
 
     @remote_api_endpoint("snapshot/missing")
-    def snapshot_missing(self, snapshots):
+    def snapshot_missing(self, snapshots: List[Sha1Git]) -> Iterable[Sha1Git]:
         """List snapshots missing from storage
 
         Args:
-            snapshots (iterable): an iterable of snapshot ids
+            snapshots: snapshot ids
 
         Yields:
             missing snapshot ids
diff --git a/swh/storage/storage.py b/swh/storage/storage.py
--- a/swh/storage/storage.py
+++ b/swh/storage/storage.py
@@ -742,7 +742,9 @@
 
     @timed
     @db_transaction_generator()
-    def snapshot_missing(self, snapshots, db=None, cur=None):
+    def snapshot_missing(
+        self, snapshots: List[Sha1Git], db=None, cur=None
+    ) -> Iterable[Sha1Git]:
         for obj in db.snapshot_missing_from_list(snapshots, cur):
             yield obj[0]