diff --git a/test/test_jsinterp.py b/test/test_jsinterp.py
index a1088cea49..43b1d0fdee 100644
--- a/test/test_jsinterp.py
+++ b/test/test_jsinterp.py
@@ -536,6 +536,11 @@ class TestJSInterpreter(unittest.TestCase):
             }
         ''', 31)
 
+    def test_undefined_varnames(self):
+        jsi = JSInterpreter('function f(){ var a; return [a, b]; }')
+        self._test(jsi, [JS_Undefined, JS_Undefined])
+        self.assertEqual(jsi._undefined_varnames, {'b'})
+
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/test/test_youtube_signature.py b/test/test_youtube_signature.py
index 98607df55e..4562467534 100644
--- a/test/test_youtube_signature.py
+++ b/test/test_youtube_signature.py
@@ -373,6 +373,10 @@ _NSIG_TESTS = [
         'https://www.youtube.com/s/player/e12fbea4/player_ias_tce.vflset/en_US/base.js',
         'kM5r52fugSZRAKHfo3', 'XkeRfXIPOkSwfg',
     ),
+    (
+        'https://www.youtube.com/s/player/ef259203/player_ias_tce.vflset/en_US/base.js',
+        'rPqBC01nJpqhhi2iA2U', 'hY7dbiKFT51UIA',
+    ),
 ]
 
 
diff --git a/yt_dlp/jsinterp.py b/yt_dlp/jsinterp.py
index f06d96832f..460bc2c03e 100644
--- a/yt_dlp/jsinterp.py
+++ b/yt_dlp/jsinterp.py
@@ -677,8 +677,9 @@ class JSInterpreter:
                 # Set value as JS_Undefined or its pre-existing value
                 local_vars.set_local(var, ret)
             else:
-                ret = local_vars.get(var, JS_Undefined)
-                if ret is JS_Undefined:
+                ret = local_vars.get(var, NO_DEFAULT)
+                if ret is NO_DEFAULT:
+                    ret = JS_Undefined
                     self._undefined_varnames.add(var)
             return ret, should_return