bug fix

mongodb/source/autograder.py

@@ -1,6 +1,6 @@
 import pymongo, json, re
 from bson import json_util
-print('ver: 1.5')
+print('ver: 1.51')
 # dbprep
 fsroot = '/autograder/source/'
 datasets = ['movies', 'comments']
@@ -89,6 +89,7 @@ def evaluate(query : str):
     if query.endswith(';'): query = query[:-1]
     true = True
     false = False
+    null = None
     data = list(eval(query))
     return [postproc_iter(data, n) if query else None for n in (True, False)]