diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst
index fe64daa3291d67..5470d05bb11f91 100644
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -236,8 +236,8 @@ underlying :class:`Popen` interface can be used directly.
 
     .. attribute:: returncode
 
-        Exit status of the child process.  If the process exited due to a
-        signal, this will be the negative signal number.
+        Exit status of the child process, an integer.  If the process
+        exited due to a signal, this will be the negative signal number.
 
     .. attribute:: cmd
 
@@ -260,6 +260,10 @@ underlying :class:`Popen` interface can be used directly.
     .. versionchanged:: 3.5
         *stdout* and *stderr* attributes added
 
+    .. versionchanged:: 3.16
+        The constructor now raises :exc:`TypeError` if *returncode* is
+        neither an integer nor ``None``.
+
 
 .. _frequently-used-arguments:
 
diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 6fe2ec98fb4088..7853413aff6f55 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -137,12 +137,18 @@ class CalledProcessError(SubprocessError):
       cmd, returncode, stdout, stderr, output
     """
     def __init__(self, returncode, cmd, output=None, stderr=None):
+        if returncode is not None and not isinstance(returncode, int):
+            raise TypeError(
+                "returncode must be an integer or None, not "
+                f"{type(returncode).__name__}")
         self.returncode = returncode
         self.cmd = cmd
         self.output = output
         self.stderr = stderr
 
     def __str__(self):
+        # A None returncode is falsy, so it skips the '< 0' comparison
+        # below and is formatted by the final branch instead.
         if self.returncode and self.returncode < 0:
             try:
                 return "Command %r died with %r." % (
@@ -151,8 +157,8 @@ def __str__(self):
                 return "Command %r died with unknown signal %d." % (
                         self.cmd, -self.returncode)
         else:
-            return "Command %r returned non-zero exit status %d." % (
-                    self.cmd, self.returncode)
+            return (f"Command {self.cmd!r} returned non-zero "
+                    f"exit status {self.returncode}.")
 
     @property
     def stdout(self):
diff --git a/Lib/test/test_script_helper.py b/Lib/test/test_script_helper.py
index eeea6c4842b488..3c5763b0b8374d 100644
--- a/Lib/test/test_script_helper.py
+++ b/Lib/test/test_script_helper.py
@@ -86,7 +86,7 @@ def tearDown(self):
     def test_interpreter_requires_environment_true(self, mock_check_call):
         with mock.patch.dict(os.environ):
             os.environ.pop('PYTHONHOME', None)
-            mock_check_call.side_effect = subprocess.CalledProcessError('', '')
+            mock_check_call.side_effect = subprocess.CalledProcessError(1, 'cmd')
             self.assertTrue(script_helper.interpreter_requires_environment())
             self.assertTrue(script_helper.interpreter_requires_environment())
             self.assertEqual(1, mock_check_call.call_count)
diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index d066ae85dfc51a..d3368a745fc374 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -2449,6 +2449,20 @@ def test_CalledProcessError_str(self):
         err = subprocess.CalledProcessError(-9876543, "fake cmd")
         self.assertEqual(str(err), "Command 'fake cmd' died with unknown signal 9876543.")
 
+        # None return code (must not raise TypeError from __str__ itself)
+        err = subprocess.CalledProcessError(None, "fake cmd")
+        self.assertEqual(str(err), "Command 'fake cmd' returned non-zero exit status None.")
+
+    def test_CalledProcessError_returncode_type(self):
+        # returncode must be an integer or None; other types are rejected
+        # at construction so that __str__ cannot fail later on.
+        subprocess.CalledProcessError(1, "fake cmd")
+        subprocess.CalledProcessError(None, "fake cmd")
+        for returncode in ("1", 1.5, [1], object()):
+            with self.subTest(returncode=returncode):
+                with self.assertRaises(TypeError):
+                    subprocess.CalledProcessError(returncode, "fake cmd")
+
     def test_preexec(self):
         # DISCLAIMER: Setting environment variables is *not* a good use
         # of a preexec_fn.  This is merely a test.
diff --git a/Misc/NEWS.d/next/Library/2026-07-22-12-00-00.gh-issue-153970.Kq7Wn3.rst b/Misc/NEWS.d/next/Library/2026-07-22-12-00-00.gh-issue-153970.Kq7Wn3.rst
new file mode 100644
index 00000000000000..044982326adc53
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-22-12-00-00.gh-issue-153970.Kq7Wn3.rst
@@ -0,0 +1,4 @@
+Calling :func:`str` on a :exc:`subprocess.CalledProcessError` whose
+:attr:`!returncode` is ``None`` no longer raises :exc:`TypeError`.  The
+constructor now also raises :exc:`TypeError` when *returncode* is neither
+an integer nor ``None``.