diff --git a/reactor-core/src/main/java/reactor/core/publisher/FluxBufferTimeout.java b/reactor-core/src/main/java/reactor/core/publisher/FluxBufferTimeout.java
index 9b2508255a..dcc6f7228f 100644
--- a/reactor-core/src/main/java/reactor/core/publisher/FluxBufferTimeout.java
+++ b/reactor-core/src/main/java/reactor/core/publisher/FluxBufferTimeout.java
@@ -694,9 +694,11 @@ public Object scanUnsafe(Attr key) {
 		@Override
 		public void onNext(final T value) {
 			int index;
+			boolean flush;
 			for(;;){
 				index = this.index + 1;
-				if(INDEX.compareAndSet(this, index - 1, index)){
+				flush = index % batchSize == 0;
+				if(INDEX.compareAndSet(this, index - 1, flush ? 0 : index)){
 					break;
 				}
 			}
@@ -715,8 +717,7 @@ public void onNext(final T value) {
 
 			nextCallback(value);
 
-			if (this.index % batchSize == 0) {
-				this.index = 0;
+			if (flush) {
 				if (timespanRegistration != null) {
 					timespanRegistration.dispose();
 					timespanRegistration = null;
diff --git a/reactor-core/src/test/java/reactor/core/publisher/FluxBufferTimeoutTest.java b/reactor-core/src/test/java/reactor/core/publisher/FluxBufferTimeoutTest.java
index 9d703b4daa..e750a5df96 100644
--- a/reactor-core/src/test/java/reactor/core/publisher/FluxBufferTimeoutTest.java
+++ b/reactor-core/src/test/java/reactor/core/publisher/FluxBufferTimeoutTest.java
@@ -24,11 +24,13 @@
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Consumer;
 
+import org.assertj.core.api.Assertions;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
 import org.reactivestreams.Subscription;
@@ -310,31 +312,54 @@ public void scanSubscriberCancelled() {
 
 	@Test
 	public void flushShouldNotRaceWithNext() {
-		Set<Integer> seen = new HashSet<>();
-		Consumer<List<Integer>> consumer = integers -> {
-			for (Integer i : integers) {
-				if (!seen.add(i)) {
-					throw new IllegalStateException("Duplicate! " + i);
+		for (int i = 0; i < 100; i++) {
+			AtomicInteger caller = new AtomicInteger();
+			AtomicBoolean stop = new AtomicBoolean();
+			Set<Integer> seen = new HashSet<>();
+			Consumer<List<Integer>> consumer = integers -> {
+				RuntimeException ex = new RuntimeException(integers.toString());
+				if (caller.getAndIncrement() == 0) {
+					for (Integer value : integers) {
+						if (!seen.add(value)) {
+							throw new IllegalStateException("Duplicate! " + value);
+						}
+					}
+
+					if (caller.decrementAndGet() != 0) {
+						stop.set(true);
+						throw ex;
+					}
 				}
+				else {
+					stop.set(true);
+					throw ex;
+				}
+			};
+			CoreSubscriber<List<Integer>> actual =
+					new LambdaSubscriber<>(consumer, null, null, null);
+
+			FluxBufferTimeout.BufferTimeoutSubscriber<Integer, List<Integer>> test =
+					new FluxBufferTimeout.BufferTimeoutSubscriber<Integer, List<Integer>>(
+							actual,
+							3,
+							1000,
+							TimeUnit.MILLISECONDS,
+							Schedulers.boundedElastic()
+							          .createWorker(),
+							ArrayList::new);
+			test.onSubscribe(Operators.emptySubscription());
+
+			AtomicInteger counter = new AtomicInteger();
+			for (int j = 0; j < 500; j++) {
+				RaceTestUtils.race(() -> test.onNext(counter.getAndIncrement()), test.flushTask);
+				Assertions.assertThat(stop).isFalse();
 			}
-		};
-		CoreSubscriber<List<Integer>> actual = new LambdaSubscriber<>(consumer, null, null, null);
-
-		FluxBufferTimeout.BufferTimeoutSubscriber<Integer, List<Integer>> test = new FluxBufferTimeout.BufferTimeoutSubscriber<Integer, List<Integer>>(
-				actual, 3, 1000, TimeUnit.MILLISECONDS, Schedulers.boundedElastic().createWorker(), ArrayList::new);
-		test.onSubscribe(Operators.emptySubscription());
-
-		AtomicInteger counter = new AtomicInteger();
-		for (int i = 0; i < 500; i++) {
-			RaceTestUtils.race(
-					() -> test.onNext(counter.getAndIncrement()),
-					() -> test.flushCallback(null)
-			);
-		}
 
-		test.onComplete();
+			test.onComplete();
 
-		assertThat(seen.size()).isEqualTo(500);
+			assertThat(seen.size()).as(() -> seen.size() + " " + seen.toString())
+			                       .isEqualTo(500);
+		}
 	}
 
 	//see https://github.com/reactor/reactor-core/issues/1247