diff --git a/src/Bundle/ChillActivityBundle/Export/Aggregator/ACPAggregators/ByActivityNumberAggregator.php b/src/Bundle/ChillActivityBundle/Export/Aggregator/ACPAggregators/ByActivityNumberAggregator.php
index e304a9d7f..125b197dc 100644
--- a/src/Bundle/ChillActivityBundle/Export/Aggregator/ACPAggregators/ByActivityNumberAggregator.php
+++ b/src/Bundle/ChillActivityBundle/Export/Aggregator/ACPAggregators/ByActivityNumberAggregator.php
@@ -27,7 +27,8 @@ class ByActivityNumberAggregator implements AggregatorInterface
     public function alterQuery(QueryBuilder $qb, $data, \Chill\MainBundle\Export\ExportGenerationContext $exportGenerationContext): void
     {
         $qb
-            ->addSelect('(SELECT COUNT(activity.id) FROM '.Activity::class.' activity WHERE activity.accompanyingPeriod = acp) AS activity_by_number_aggregator')
+            // Use a distinct alias inside the subquery to avoid colliding with the root alias "activity"
+            ->addSelect('(SELECT COUNT(agg_activity.id) FROM '.Activity::class.' agg_activity WHERE agg_activity.accompanyingPeriod = acp) AS activity_by_number_aggregator')
             ->addGroupBy('activity_by_number_aggregator');
     }