diff --git a/sdk-tests/src/test/java/io/dapr/it/pubsub/http/SubscriberController.java b/sdk-tests/src/test/java/io/dapr/it/pubsub/http/SubscriberController.java
index cb9d3a2f9..87bdf9872 100644
--- a/sdk-tests/src/test/java/io/dapr/it/pubsub/http/SubscriberController.java
+++ b/sdk-tests/src/test/java/io/dapr/it/pubsub/http/SubscriberController.java
@@ -27,7 +27,7 @@ import java.util.function.BiFunction;
 @RestController
 public class SubscriberController {
 
-  private final Map<String, List<CloudEvent<?>>> messagesByTopic = new HashMap<>();
+  private final Map<String, List<CloudEvent<?>>> messagesByTopic = Collections.synchronizedMap(new HashMap<>());
 
   @GetMapping(path = "/messages/{topic}")
   public List<CloudEvent<?>> getMessagesByTopic(@PathVariable("topic") String topic) {