diff --git a/kitty_tests/prewarm.py b/kitty_tests/prewarm.py
index 4cd4c7309..5f1a4a6c2 100644
--- a/kitty_tests/prewarm.py
+++ b/kitty_tests/prewarm.py
@@ -50,13 +50,11 @@ import os, json; from kitty.utils import *; from kitty.fast_data_types import ge
         'pid': os.getpid(),
         'font_family': get_options().font_family,
         'stdin': sys.stdin.read(),
-
-        'done': 'hello',
-        }, indent=2))"""], cwd=cwd, env=env, stdin_data=stdin_data)
+        }, indent=2), "ALL_OUTPUT_PRESENT", sep="")"""], cwd=cwd, env=env, stdin_data=stdin_data)
         self.assertFalse(pty.screen_contents().strip())
         p.mark_child_as_ready(child.child_id)
-        pty.wait_till(lambda: 'hello' in pty.screen_contents())
-        data = json.loads(pty.screen_contents())
+        pty.wait_till(lambda: 'ALL_OUTPUT_PRESENT' in pty.screen_contents())
+        data = json.JSONDecoder().raw_decode(pty.screen_contents())[0]
         self.ae(data['cols'], cols)
         self.assertTrue(data['cterm'])
         self.ae(data['ttyname'], ttyname)