diff --git a/quickstats/concurrent/parameterised_runner.py b/quickstats/concurrent/parameterised_runner.py
index 531ca677d55be85cb2b25bbf5301d4072003b870..355655e8b8171aca2c4fd6852b36e5a370de59e2 100644
--- a/quickstats/concurrent/parameterised_runner.py
+++ b/quickstats/concurrent/parameterised_runner.py
@@ -1,5 +1,6 @@
 from typing import Optional, List, Dict, Union, Tuple
 import os
+import json
 
 import ROOT