diff --git a/raft/src/main/java/org/apache/kafka/raft/LeaderState.java b/raft/src/main/java/org/apache/kafka/raft/LeaderState.java
index eb5f8cb3d87..ff74602217b 100644
--- a/raft/src/main/java/org/apache/kafka/raft/LeaderState.java
+++ b/raft/src/main/java/org/apache/kafka/raft/LeaderState.java
@@ -156,7 +156,10 @@ public class LeaderState<T> implements EpochState {
                 "Current fetched voters are {}, and voters are {}",
                 checkQuorumTimeoutMs,
                 fetchedVoters,
-                voterStates.values().stream().map(voter -> voter.replicaKey)
+                voterStates.values()
+                    .stream()
+                    .map(voter -> voter.replicaKey)
+                    .collect(Collectors.toUnmodifiableSet())
             );
         }
         return remainingMs;
diff --git a/raft/src/main/java/org/apache/kafka/raft/ReplicaKey.java b/raft/src/main/java/org/apache/kafka/raft/ReplicaKey.java
index a1acc39d57c..f25a1d55ba4 100644
--- a/raft/src/main/java/org/apache/kafka/raft/ReplicaKey.java
+++ b/raft/src/main/java/org/apache/kafka/raft/ReplicaKey.java
@@ -70,7 +70,7 @@ public final class ReplicaKey implements Comparable<ReplicaKey> {
 
     @Override
     public String toString() {
-        return String.format("ReplicaKey(id=%d, directoryId=%s)", id, directoryId);
+        return String.format("ReplicaKey(id=%d, directoryId=%s)", id, directoryId.map(Uuid::toString).orElse("<undefined>"));
     }
 
     public static ReplicaKey of(int id, Uuid directoryId) {