diff --git a/src/AggregateFunctions/AggregateFunctionTopK.h b/src/AggregateFunctions/AggregateFunctionTopK.h
index f77fc482685..791df21d1a7 100644
--- a/src/AggregateFunctions/AggregateFunctionTopK.h
+++ b/src/AggregateFunctions/AggregateFunctionTopK.h
@@ -64,7 +64,10 @@ public:
 
     void merge(AggregateDataPtr place, ConstAggregateDataPtr rhs, Arena *) const override
     {
-        this->data(place).value.merge(this->data(rhs).value);
+        auto & set = this->data(place).value;
+        if (set.capacity() != reserved)
+            set.resize(reserved);
+        set.merge(this->data(rhs).value);
     }
 
     void serialize(ConstAggregateDataPtr place, WriteBuffer & buf) const override
@@ -197,7 +200,10 @@ public:
 
     void merge(AggregateDataPtr place, ConstAggregateDataPtr rhs, Arena *) const override
     {
-        this->data(place).value.merge(this->data(rhs).value);
+        auto & set = this->data(place).value;
+        if (set.capacity() != reserved)
+            set.resize(reserved);
+        set.merge(this->data(rhs).value);
     }
 
     void insertResultInto(AggregateDataPtr place, IColumn & to, Arena *) const override
diff --git a/tests/queries/0_stateless/01409_topK_merge.reference b/tests/queries/0_stateless/01409_topK_merge.reference
new file mode 100644
index 00000000000..69db2381018
--- /dev/null
+++ b/tests/queries/0_stateless/01409_topK_merge.reference
@@ -0,0 +1,6 @@
+AggregateFunctionTopK
+20
+20
+AggregateFunctionTopKGenericData
+20
+20
diff --git a/tests/queries/0_stateless/01409_topK_merge.sql b/tests/queries/0_stateless/01409_topK_merge.sql
new file mode 100644
index 00000000000..5ac7c350093
--- /dev/null
+++ b/tests/queries/0_stateless/01409_topK_merge.sql
@@ -0,0 +1,13 @@
+drop table if exists data_01409;
+create table data_01409 engine=Memory as select * from numbers(20);
+
+-- easier to check merging via distributed tables
+-- but can be done vai topKMerge(topKState()) as well
+
+select 'AggregateFunctionTopK';
+select length(topK(20)(number)) from remote('127.{1,1}', currentDatabase(), data_01409);
+select length(topKWeighted(20)(number, 1)) from remote('127.{1,1}', currentDatabase(), data_01409);
+
+select 'AggregateFunctionTopKGenericData';
+select length(topK(20)((number, ''))) from remote('127.{1,1}', currentDatabase(), data_01409);
+select length(topKWeighted(20)((number, ''), 1)) from remote('127.{1,1}', currentDatabase(), data_01409);