diff --git a/weed/mq/sub_coordinator/inflight_message_tracker.go b/weed/mq/sub_coordinator/inflight_message_tracker.go
index 7a72d2eef..f8effef95 100644
--- a/weed/mq/sub_coordinator/inflight_message_tracker.go
+++ b/weed/mq/sub_coordinator/inflight_message_tracker.go
@@ -142,9 +142,9 @@ func (rb *RingBuffer) AckTimestamp(timestamp int64) {
 	// Remove all the continuously acknowledged timestamps from the buffer
 	startPos := (rb.head + len(rb.buffer) - rb.size) % len(rb.buffer)
 	for i := 0; i < len(rb.buffer) && rb.buffer[(startPos+i)%len(rb.buffer)].Acked; i++ {
-		rb.size--
 		t := rb.buffer[(startPos+i)%len(rb.buffer)]
 		if rb.maxAllAckedTs < t.Timestamp {
+			rb.size--
 			rb.maxAllAckedTs = t.Timestamp
 		}
 	}
diff --git a/weed/mq/sub_coordinator/inflight_message_tracker_test.go b/weed/mq/sub_coordinator/inflight_message_tracker_test.go
index 9f62f80ad..5b7a1bdd8 100644
--- a/weed/mq/sub_coordinator/inflight_message_tracker_test.go
+++ b/weed/mq/sub_coordinator/inflight_message_tracker_test.go
@@ -117,3 +117,17 @@ func TestInflightMessageTracker3(t *testing.T) {
 	assert.Equal(t, int64(7), tracker.GetOldestAckedTimestamp())
 
 }
+
+func TestInflightMessageTracker4(t *testing.T) {
+	// Initialize an InflightMessageTracker with initial capacity 1
+	tracker := NewInflightMessageTracker(1)
+
+	tracker.EnflightMessage([]byte("1"), int64(1))
+	tracker.EnflightMessage([]byte("2"), int64(2))
+	assert.True(t, tracker.AcknowledgeMessage([]byte("1"), int64(1)))
+	assert.True(t, tracker.AcknowledgeMessage([]byte("2"), int64(2)))
+	tracker.EnflightMessage([]byte("3"), int64(3))
+	assert.True(t, tracker.AcknowledgeMessage([]byte("3"), int64(3)))
+	assert.Equal(t, int64(3), tracker.GetOldestAckedTimestamp())
+
+}