diff --git a/server/model/schema.py b/server/model/schema.py
index cccf26f2..70ec87b1 100644
--- a/server/model/schema.py
+++ b/server/model/schema.py
@@ -22,7 +22,7 @@ def processor(value):
 
     def result_processor(self, dialect, coltype):
         def processor(value):
-            if not isinstance(value, bytes):
+            if value and not isinstance(value, bytes):
                 value = bytes(value)
             return str(bson.ObjectId(value)) if bson.ObjectId.is_valid(value) else value