diff --git a/test/fiber/scheduler.rb b/test/fiber/scheduler.rb
index 6e04409c6e..408e0e53e5 100644
--- a/test/fiber/scheduler.rb
+++ b/test/fiber/scheduler.rb
@@ -101,6 +101,8 @@ class Scheduler
   end
 
   def close
+    raise "Scheduler already closed!" if @closed
+
     self.run
   ensure
     @closed = true