diff --git a/lib/debug/session.rb b/lib/debug/session.rb
index 3ece5aacb..3f9541636 100644
--- a/lib/debug/session.rb
+++ b/lib/debug/session.rb
@@ -177,7 +177,11 @@ def activate ui = nil, on_fork: false
       @ui.activate self, on_fork: on_fork
 
       q = Queue.new
+      first_q = Queue.new
       @session_server = Thread.new do
+        # make sure `@session_server` is assigned
+        first_q.pop; first_q = nil
+
         Thread.current.name = 'DEBUGGER__::SESSION@server'
         Thread.current.abort_on_exception = true
 
@@ -204,6 +208,7 @@ def activate ui = nil, on_fork: false
         q << true
         session_server_main
       end
+      first_q << :ok
 
       q.pop
     end