diff --git a/test/irb/helper.rb b/test/irb/helper.rb
index 1614b42ad..2acc465a7 100644
--- a/test/irb/helper.rb
+++ b/test/irb/helper.rb
@@ -121,7 +121,9 @@ def run_ruby_file(&block)
       @envs["XDG_CONFIG_HOME"] ||= tmp_dir
       @envs["IRBRC"] = nil unless @envs.key?("IRBRC")
 
-      PTY.spawn(@envs.merge("TERM" => "dumb"), *cmd) do |read, write, pid|
+      envs_for_spawn = @envs.merge('TERM' => 'dumb', 'TEST_IRB_FORCE_INTERACTIVE' => 'true')
+
+      PTY.spawn(envs_for_spawn, *cmd) do |read, write, pid|
         Timeout.timeout(TIMEOUT_SEC) do
           while line = safe_gets(read)
             lines << line