diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index d1da0258..47704723 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -92,8 +92,8 @@ jobs:
           export path_to_file=$(find dist -type f -name "typing_extensions-*.tar.gz")
           echo "::notice::Unpacking source distribution: $path_to_file"
           tar xzf $path_to_file -C dist/
-          cd ${path_to_file%.tar.gz}
-          python src/test_typing_extensions.py
+          cd ${path_to_file%.tar.gz}/src
+          python test_typing_extensions.py
 
   test-sdist-installed:
     name: Test installed source distribution
diff --git a/CHANGELOG.md b/CHANGELOG.md
index d1ac543b..f315d1ff 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,11 @@
+# Unreleased
+
+- Fix incorrect behaviour of `typing_extensions.ParamSpec` on Python 3.8 and
+  3.9 that meant that
+  `isinstance(typing_extensions.ParamSpec("P"), typing.TypeVar)` would have a
+  different result in some situations depending on whether or not a profiling
+  function had been set using `sys.setprofile`. Patch by Alex Waygood.
+
 # Release 4.12.0rc1 (May 16, 2024)
 
 This release focuses on compatibility with the upcoming release of
diff --git a/src/test_typing_extensions.py b/src/test_typing_extensions.py
index 5d37b5bf..080c0f7c 100644
--- a/src/test_typing_extensions.py
+++ b/src/test_typing_extensions.py
@@ -5020,6 +5020,48 @@ def test_eq(self):
         # won't be the same.
         self.assertNotEqual(hash(ParamSpec('P')), hash(P))
 
+    def test_isinstance_results_unaffected_by_presence_of_tracing_function(self):
+        # See https://github.com/python/typing_extensions/issues/318
+
+        code = textwrap.dedent(
+            """\
+            import sys, typing
+
+            def trace_call(*args):
+                return trace_call
+
+            def run():
+                sys.modules.pop("typing_extensions", None)
+                from typing_extensions import ParamSpec
+                return isinstance(ParamSpec("P"), typing.TypeVar)
+
+            isinstance_result_1 = run()
+            sys.setprofile(trace_call)
+            isinstance_result_2 = run()
+            sys.stdout.write(f"{isinstance_result_1} {isinstance_result_2}")
+            """
+        )
+
+        # Run this in an isolated process or it pollutes the environment
+        # and makes other tests fail:
+        try:
+            proc = subprocess.run(
+                [sys.executable, "-c", code], check=True, capture_output=True, text=True,
+            )
+        except subprocess.CalledProcessError as exc:
+            print("stdout", exc.stdout, sep="\n")
+            print("stderr", exc.stderr, sep="\n")
+            raise
+
+        # Sanity checks that assert the test is working as expected
+        self.assertIsInstance(proc.stdout, str)
+        result1, result2 = proc.stdout.split(" ")
+        self.assertIn(result1, {"True", "False"})
+        self.assertIn(result2, {"True", "False"})
+
+        # The actual test:
+        self.assertEqual(result1, result2)
+
 
 class ConcatenateTests(BaseTestCase):
     def test_basics(self):
diff --git a/src/typing_extensions.py b/src/typing_extensions.py
index d549fb69..57e59a8b 100644
--- a/src/typing_extensions.py
+++ b/src/typing_extensions.py
@@ -1711,7 +1711,7 @@ def kwargs(self):
 
         def __init__(self, name, *, bound=None, covariant=False, contravariant=False,
                      infer_variance=False, default=NoDefault):
-            super().__init__([self])
+            list.__init__(self, [self])
             self.__name__ = name
             self.__covariant__ = bool(covariant)
             self.__contravariant__ = bool(contravariant)