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
@@ -247,11 +247,18 @@
                 results.append(Content(**row_d))
         return results
 
-    def content_missing(self, content, key_hash="sha1"):
-        for cont in content:
-            res = self.content_find(cont)
+    def content_missing(
+        self, contents: List[Dict[str, Any]], key_hash: str = "sha1"
+    ) -> Iterable[bytes]:
+        if key_hash not in DEFAULT_ALGORITHMS:
+            raise StorageArgumentException(
+                "key_hash should be one of {','.join(DEFAULT_ALGORITHMS)}"
+            )
+
+        for content in contents:
+            res = self.content_find(content)
             if not res:
-                yield cont[key_hash]
+                yield content[key_hash]
 
     def content_missing_per_sha1(self, contents):
         return self.content_missing([{"sha1": c for c in contents}])
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
@@ -345,13 +345,20 @@
         keys = list(set.intersection(*found))
         return [self._contents[key] for key in keys]
 
-    def content_missing(self, content, key_hash="sha1"):
-        for cont in content:
-            for (algo, hash_) in cont.items():
+    def content_missing(
+        self, contents: List[Dict[str, Any]], key_hash: str = "sha1"
+    ) -> Iterable[bytes]:
+        if key_hash not in DEFAULT_ALGORITHMS:
+            raise StorageArgumentException(
+                "key_hash should be one of {','.join(DEFAULT_ALGORITHMS)}"
+            )
+
+        for content in contents:
+            for (algo, hash_) in content.items():
                 if algo not in DEFAULT_ALGORITHMS:
                     continue
                 if hash_ not in self._content_indexes.get(algo, []):
-                    yield cont[key_hash]
+                    yield content[key_hash]
                     break
 
     def content_missing_per_sha1(self, contents):
diff --git a/swh/storage/interface.py b/swh/storage/interface.py
--- a/swh/storage/interface.py
+++ b/swh/storage/interface.py
@@ -247,26 +247,24 @@
         ...
 
     @remote_api_endpoint("content/missing")
-    def content_missing(self, content, key_hash="sha1"):
+    def content_missing(
+        self, contents: List[Dict[str, Any]], key_hash: str = "sha1"
+    ) -> Iterable[bytes]:
         """List content missing from storage
 
         Args:
-            content ([dict]): iterable of dictionaries whose keys are
-                              either 'length' or an item of
-                              :data:`swh.model.hashutil.ALGORITHMS`;
-                              mapped to the corresponding checksum
-                              (or length).
-
-            key_hash (str): name of the column to use as hash id
-                            result (default: 'sha1')
-
-        Returns:
-            iterable ([bytes]): missing content ids (as per the
-            key_hash column)
+            content: iterable of dictionaries whose keys are either 'length' or an item
+                of :data:`swh.model.hashutil.ALGORITHMS`; mapped to the
+                corresponding checksum (or length).
+            key_hash: name of the column to use as hash id result (default: 'sha1')
 
         Raises:
+            StorageArgumentException when key_hash is unknown.
             TODO: an exception when we get a hash collision.
 
+        Returns:
+            iterable of missing content ids (as per the `key_hash` column)
+
         """
         ...
 
diff --git a/swh/storage/storage.py b/swh/storage/storage.py
--- a/swh/storage/storage.py
+++ b/swh/storage/storage.py
@@ -338,18 +338,18 @@
 
     @timed
     @db_transaction_generator()
-    def content_missing(self, content, key_hash="sha1", db=None, cur=None):
-        keys = db.content_hash_keys
-
-        if key_hash not in keys:
-            raise StorageArgumentException("key_hash should be one of %s" % keys)
+    def content_missing(
+        self, contents: List[Dict[str, Any]], key_hash: str = "sha1", db=None, cur=None
+    ) -> Iterable[bytes]:
+        if key_hash not in DEFAULT_ALGORITHMS:
+            raise StorageArgumentException(
+                "key_hash should be one of {','.join(DEFAULT_ALGORITHMS)}"
+            )
 
+        keys = db.content_hash_keys
         key_hash_idx = keys.index(key_hash)
 
-        if not content:
-            return
-
-        for obj in db.content_missing_from_list(content, cur):
+        for obj in db.content_missing_from_list(contents, cur):
             yield obj[key_hash_idx]
 
     @timed