diff --git a/internal/mainthread/mainthread.go b/internal/mainthread/mainthread.go
index 184b870b4..6491e3ee3 100644
--- a/internal/mainthread/mainthread.go
+++ b/internal/mainthread/mainthread.go
@@ -45,6 +45,8 @@ func Loop(ch <-chan error) error {
 }
 
 // Run calls f on the main thread.
+//
+// Do not call this from the main thread. This would block forever.
 func Run(f func() error) error {
 	if atomic.LoadInt32(&started) == 0 {
 		// TODO: This can reach from other goroutine before Loop is called (#809).