diff --git a/negi/string_function.py b/negi/string_function.py
index aedcafd..963d70e 100644
--- a/negi/string_function.py
+++ b/negi/string_function.py
@@ -1,6 +1,7 @@
 _sandbox_template0 = '''\
 import random
 import re
+import json
 import numpy as np
 
 def _string_function(a, b, c):