diff --git a/src/Processors/QueryPlan/IQueryPlanStep.h b/src/Processors/QueryPlan/IQueryPlanStep.h
index 938ddda81b1..4f9e4d3268c 100644
--- a/src/Processors/QueryPlan/IQueryPlanStep.h
+++ b/src/Processors/QueryPlan/IQueryPlanStep.h
@@ -14,8 +14,8 @@ class DataStream
 public:
     Block header;
 
-    NameSet distinct_columns;
-    NameSet local_distinct_columns; /// Those columns are distinct in separate thread, but not in general.
+    NameSet distinct_columns = {};
+    NameSet local_distinct_columns = {}; /// Those columns are distinct in separate thread, but not in general.
 
     /// Things which may be added:
     /// * sort description