diff --git a/session.go b/session.go
index c9e3743b..ce9b40f5 100644
--- a/session.go
+++ b/session.go
@@ -933,6 +933,12 @@ sendLoop:
 		case ackhandler.SendNone:
 			break sendLoop
 		case ackhandler.SendAck:
+			// If we already sent packets, and the send mode switches to SendAck,
+			// we've just become congestion limited.
+			// There's no need to try to send an ACK at this moment.
+			if numPacketsSent > 0 {
+				return nil
+			}
 			// We can at most send a single ACK only packet.
 			// There will only be a new ACK after receiving new packets.
 			// SendAck is only returned when we're congestion limited, so we don't need to set the pacingt timer.
diff --git a/session_test.go b/session_test.go
index 06167ad5..26cc4650 100644
--- a/session_test.go
+++ b/session_test.go
@@ -890,6 +890,10 @@ var _ = Describe("Session", func() {
 			sph.EXPECT().TimeUntilSend().Return(time.Now())
 			sph.EXPECT().SendMode().Return(ackhandler.SendAny)
 			sph.EXPECT().SendMode().Return(ackhandler.SendAck)
+			rph := mockackhandler.NewMockReceivedPacketHandler(mockCtrl)
+			rph.EXPECT().GetAlarmTimeout().Return(time.Now().Add(time.Hour)).Times(2)
+			rph.EXPECT().GetAckFrame()
+			sess.receivedPacketHandler = rph
 			done := make(chan struct{})
 			go func() {
 				defer GinkgoRecover()