diff --git a/js.go b/js.go
index c0ae58e2b..6683edbcb 100644
--- a/js.go
+++ b/js.go
@@ -718,6 +718,10 @@ func (m *Msg) checkReply() (*js, bool, error) {
 	}
 	sub := m.Sub
 	sub.mu.Lock()
+	if sub.jsi == nil {
+		sub.mu.Unlock()
+		return nil, false, ErrNotJSMessage
+	}
 	js := sub.jsi.js
 	isPullMode := sub.jsi.pull > 0
 	sub.mu.Unlock()
diff --git a/test/js_test.go b/test/js_test.go
index b0b0eed6a..aa12f0fcd 100644
--- a/test/js_test.go
+++ b/test/js_test.go
@@ -472,6 +472,27 @@ func TestJetStreamSubscribe(t *testing.T) {
 	}
 }
 
+func TestAckForNonJetStream(t *testing.T) {
+	s := RunBasicJetStreamServer()
+	defer s.Shutdown()
+
+	nc, err := nats.Connect(s.ClientURL())
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	defer nc.Close()
+
+	sub, _ := nc.SubscribeSync("foo")
+	nc.PublishRequest("foo", "_INBOX_", []byte("OK"))
+	m, err := sub.NextMsg(time.Second)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	if err := m.Ack(); err != nats.ErrNotJSMessage {
+		t.Fatalf("Expected an error of '%v', got '%v'", nats.ErrNotJSMessage, err)
+	}
+}
+
 // TODO(dlc) - fill out with more stuff.
 func TestJetStreamManagement(t *testing.T) {
 	s := RunBasicJetStreamServer()