diff --git a/swh/storage/filter.py b/swh/storage/filter.py
--- a/swh/storage/filter.py
+++ b/swh/storage/filter.py
@@ -49,7 +49,8 @@
         contents = list(content)
         contents_to_add = self._filter_missing_skipped_contents(contents)
         return self.storage.skipped_content_add(
-            x for x in contents if x['sha1_git'] in contents_to_add
+            x for x in contents
+            if x.get('sha1_git') is None or x['sha1_git'] in contents_to_add
         )
 
     def directory_add(self, directories: Iterable[Dict]) -> Dict:
@@ -106,7 +107,8 @@
         objects_seen = self.objects_seen['skipped_content']
         missing_hashes = []
         for hashes in content_hashes:
-            if hashes['sha1_git'] in objects_seen:
+            if hashes.get('sha1_git') is None \
+                    or hashes['sha1_git'] in objects_seen:
                 continue
             objects_seen.add(hashes['sha1_git'])
             missing_hashes.append(hashes)
diff --git a/swh/storage/tests/test_filter.py b/swh/storage/tests/test_filter.py
--- a/swh/storage/tests/test_filter.py
+++ b/swh/storage/tests/test_filter.py
@@ -59,6 +59,32 @@
     }
 
 
+def test_filtering_proxy_storage_skipped_content_missing_sha1_git(sample_data):
+    sample_content = sample_data['skipped_content'][0]
+    sample_content2 = sample_data['skipped_content'][1]
+    storage = FilteringProxyStorage(storage=storage_config)
+
+    sample_content['sha1_git'] = sample_content2['sha1_git'] = None
+    content = next(storage.skipped_content_missing([sample_content]))
+    assert content['sha1'] == sample_content['sha1']
+
+    s = storage.skipped_content_add([sample_content])
+    assert s == {
+        'skipped_content:add': 1,
+    }
+
+    content = list(storage.skipped_content_missing([sample_content]))
+    assert content == []
+
+    s = storage.skipped_content_add([sample_content2])
+    assert s == {
+        'skipped_content:add': 1,
+    }
+
+    content = list(storage.skipped_content_missing([sample_content2]))
+    assert content == []
+
+
 def test_filtering_proxy_storage_revision(sample_data):
     sample_revision = sample_data['revision'][0]
     storage = FilteringProxyStorage(storage=storage_config)