Require explicit modifiable list.

1 files changed, 2 insertions, 1 deletions

diff --git a/container-search/src/main/java/com/yahoo/search/federation/FederationResult.java b/container-search/src/main/java/com/yahoo/search/federation/FederationResult.java
index 9c2a90034a2..d85a4c25413 100644
--- a/container-search/src/main/java/com/yahoo/search/federation/FederationResult.java
+++ b/container-search/src/main/java/com/yahoo/search/federation/FederationResult.java
@@ -32,7 +32,8 @@ class FederationResult {
         this.targetResults = targetResults;
 
         if (targetResults.stream().anyMatch(TargetResult::isMandatory))
-            targetsToWaitFor = targetResults.stream().filter(TargetResult::isMandatory).toList();
+            targetsToWaitFor = targetResults.stream().filter(TargetResult::isMandatory)
+                    .collect(Collectors.toCollection(ArrayList::new));
         else
             targetsToWaitFor = new ArrayList<>(targetResults);
     }