diff --git a/interop-testing/src/main/java/io/grpc/testing/integration/TestServiceImpl.java b/interop-testing/src/main/java/io/grpc/testing/integration/TestServiceImpl.java
index 923aca75f3..0b18494fc0 100644
--- a/interop-testing/src/main/java/io/grpc/testing/integration/TestServiceImpl.java
+++ b/interop-testing/src/main/java/io/grpc/testing/integration/TestServiceImpl.java
@@ -202,10 +202,7 @@ public class TestServiceImpl extends TestServiceGrpc.TestServiceImplBase {
   public StreamObserver<Messages.StreamingOutputCallRequest> fullDuplexCall(
       final StreamObserver<Messages.StreamingOutputCallResponse> responseObserver) {
     final ResponseDispatcher dispatcher = new ResponseDispatcher(responseObserver);
-    ServerCallStreamObserver<Messages.StreamingOutputCallResponse> autoUnlockResponseObserver =
-        (ServerCallStreamObserver<Messages.StreamingOutputCallResponse>) responseObserver;
-
-    class MayBlockStreamObserver implements StreamObserver<StreamingOutputCallRequest> {
+    return new StreamObserver<StreamingOutputCallRequest>() {
       boolean oobTestLocked;
 
       @Override
@@ -217,7 +214,7 @@ public class TestServiceImpl extends TestServiceGrpc.TestServiceImplBase {
             try {
               lock.acquire();
             } catch (InterruptedException ex) {
-              autoUnlockResponseObserver.onError(new StatusRuntimeException(
+              responseObserver.onError(new StatusRuntimeException(
                   Status.ABORTED.withDescription("server service interrupted").withCause(ex)));
               return;
             }
@@ -237,6 +234,10 @@ public class TestServiceImpl extends TestServiceGrpc.TestServiceImplBase {
 
       @Override
       public void onCompleted() {
+        if (oobTestLocked) {
+          lock.release();
+          oobTestLocked = false;
+        }
         if (!dispatcher.isCancelled()) {
           // Tell the dispatcher that all input has been received.
           dispatcher.completeInput();
@@ -245,21 +246,13 @@ public class TestServiceImpl extends TestServiceGrpc.TestServiceImplBase {
 
       @Override
       public void onError(Throwable cause) {
-        dispatcher.onError(cause);
-      }
-
-      void cleanup() {
         if (oobTestLocked) {
           lock.release();
           oobTestLocked = false;
         }
+        dispatcher.onError(cause);
       }
-    }
-
-    MayBlockStreamObserver mayBlockObserver = new MayBlockStreamObserver();
-    autoUnlockResponseObserver.setOnCancelHandler(mayBlockObserver::cleanup);
-    autoUnlockResponseObserver.setOnCloseHandler(mayBlockObserver::cleanup);
-    return mayBlockObserver;
+    };
   }
 
   /**