diff --git a/src/edu/washington/escience/myria/expression/evaluate/PythonUDFEvaluator.java b/src/edu/washington/escience/myria/expression/evaluate/PythonUDFEvaluator.java
index 863d85d5c..e7b3b5b0c 100644
--- a/src/edu/washington/escience/myria/expression/evaluate/PythonUDFEvaluator.java
+++ b/src/edu/washington/escience/myria/expression/evaluate/PythonUDFEvaluator.java
@@ -115,6 +115,7 @@ public PythonUDFEvaluator(
     pyWorker = new PythonWorker();
     pyWorker.sendCodePickle(fs.getBinary(), columnIdxs.length, outputType, isMultiValued);
     buffer = new TupleBuffer(stateSchema);
+    groups = new IntObjectHashMap<>();
   }
 
   /**