diff --git a/boards/management/commands/invalidate_sync_cache.py b/boards/management/commands/invalidate_sync_cache.py
--- a/boards/management/commands/invalidate_sync_cache.py
+++ b/boards/management/commands/invalidate_sync_cache.py
@@ -12,9 +12,11 @@ class Command(BaseCommand):
     @transaction.atomic
     def handle(self, *args, **options):
         count = 0
-        for global_id in GlobalId.objects.all():
+        for global_id in GlobalId.objects.exclude(content__isnull=True).exclude(
+                content=''):
             if global_id.is_local() and global_id.content is not None:
                 global_id.content = None
                 global_id.save()
+                global_id.signature_set.all().delete()
                 count += 1
         print('Invalidated {} caches.'.format(count))