diff --git a/internal/mainthread/mainthread.go b/internal/mainthread/mainthread.go
index c54cc618a..184b870b4 100644
--- a/internal/mainthread/mainthread.go
+++ b/internal/mainthread/mainthread.go
@@ -47,7 +47,8 @@ func Loop(ch <-chan error) error {
 // Run calls f on the main thread.
 func Run(f func() error) error {
 	if atomic.LoadInt32(&started) == 0 {
-		panic("mainthread: the mainthread loop is not started yet")
+		// TODO: This can reach from other goroutine before Loop is called (#809).
+		// panic("mainthread: the mainthread loop is not started yet")
 	}
 
 	ch := make(chan struct{})