diff --git a/zio-kafka-test/src/test/scala/zio/kafka/consumer/ConsumerSpec.scala b/zio-kafka-test/src/test/scala/zio/kafka/consumer/ConsumerSpec.scala
index 89dd0a906..a921e6048 100644
--- a/zio-kafka-test/src/test/scala/zio/kafka/consumer/ConsumerSpec.scala
+++ b/zio-kafka-test/src/test/scala/zio/kafka/consumer/ConsumerSpec.scala
@@ -648,7 +648,7 @@ object ConsumerSpec extends ZIOSpecDefaultSlf4j with KafkaRandom {
                             .tap { record =>
                               // Signal consumer 2 can start when a record is seen for every partition.
                               for {
-                                keys <- c1Keys.updateAndGet(_.appended(record.key))
+                                keys <- c1Keys.updateAndGet(_ ++ record.key)
                                 _ <- c1Started.succeed(()).when(keys.map(_.split('-')(1)).toSet.size == partitionCount)
                               } yield ()
                             }