diff --git a/Doc/library/email.compat32-message.rst b/Doc/library/email.compat32-message.rst
index 5754c2b65b239f9..b8710314aa05c7e 100644
--- a/Doc/library/email.compat32-message.rst
+++ b/Doc/library/email.compat32-message.rst
@@ -419,6 +419,19 @@ Here are the methods of the :class:`Message` class:
 
          Content-Disposition: attachment; filename*="iso-8859-1''Fu%DFballer.ppt"
 
+      When called without any *_params*, this method stores *_value* the same
+      way :meth:`__setitem__` does, so *_value* may also be an
+      :class:`~email.header.Header` instance, such as those returned by
+      :meth:`items` under a ``compat32`` policy.  Combining a
+      :class:`~email.header.Header` *_value* with *_params* raises
+      :exc:`TypeError`, since the parameters have no defined meaning for it.
+
+      .. versionchanged:: next
+         When no parameters are given, an :class:`~email.header.Header`
+         *_value* is now accepted, consistent with :meth:`__setitem__`.
+         Combining a :class:`~email.header.Header` *_value* with parameters
+         now raises a clear :exc:`TypeError` instead of an internal one.
+
 
    .. method:: replace_header(_name, _value)
 
diff --git a/Lib/email/message.py b/Lib/email/message.py
index 641fb2e944d4311..06a6baedf37c7c3 100644
--- a/Lib/email/message.py
+++ b/Lib/email/message.py
@@ -576,6 +576,21 @@ def add_header(self, _name, _value, **_params):
         msg.add_header('content-disposition', 'attachment',
                        filename='Fußballer.ppt'))
         """
+        if not _params:
+            # With no parameters, mirror __setitem__ so add_header() accepts
+            # the same values, e.g. the Header instances that items() returns
+            # under the compat32 policy.  None is coerced to '' to preserve
+            # add_header()'s historical behavior (__setitem__ would store None).
+            self[_name] = '' if _value is None else _value
+            return
+        if _value is not None and not isinstance(_value, str):
+            # Combining a non-str value (e.g. a Header instance) with
+            # parameters isn't supported: raise a clear error instead of
+            # failing inside SEMISPACE.join() below.
+            raise TypeError(
+                f'add_header() value must be a str when parameters are '
+                f'given, not {type(_value).__name__}; pass a str, or omit '
+                f'the parameters and set the header with msg[name] = value')
         parts = []
         for k, v in _params.items():
             if v is None:
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index 19555d87085e176..0d586f15ece14e5 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -839,6 +839,32 @@ def test_add_header_with_no_value(self):
         msg.add_header('X-Status', None)
         self.assertEqual('', msg['X-Status'])
 
+    def test_add_header_copies_Header_returned_by_items(self):
+        # gh-151454: items() returns (name, Header) under the compat32 policy
+        # for a header carrying 8-bit data.  Copying such headers with the
+        # bug report's loop (newmsg.add_header(h, v)) used to raise
+        # "TypeError: sequence item 0: expected str instance, Header found".
+        old = email.message_from_bytes(b'X-Ham-Report: spam \xff report\n\n')
+        name, value = old.items()[0]
+        self.assertIsInstance(value, Header)
+        msg = Message()
+        msg.add_header(name, value)
+        # The Header is stored unmodified, exactly as __setitem__ does (rather
+        # than stringified, which would lose the 8-bit bytes to U+FFFD).
+        self.assertIs(next(msg.raw_items())[1], value)
+        ref = Message()
+        ref[name] = value
+        self.assertEqual(msg.as_bytes(), ref.as_bytes())
+
+    def test_add_header_with_Header_value_and_params_raises_clear_error(self):
+        # gh-151454: a Header combined with params isn't supported (the
+        # params would have no defined meaning for it), so this should raise
+        # a clear TypeError rather than fail inside SEMISPACE.join().
+        msg = Message()
+        header = Header('spam \xff report', charset='unknown-8bit')
+        with self.assertRaisesRegex(TypeError, 'must be a str'):
+            msg.add_header('X-Ham-Report', header, foo='bar')
+
     # Issue 5871: reject an attempt to embed a header inside a header value
     # (header injection attack).
     def test_embedded_header_via_Header_rejected(self):
diff --git a/Misc/NEWS.d/next/Library/2026-06-13-15-00-38.gh-issue-151454.BQQ7up.rst b/Misc/NEWS.d/next/Library/2026-06-13-15-00-38.gh-issue-151454.BQQ7up.rst
new file mode 100644
index 000000000000000..33418824598d641
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-13-15-00-38.gh-issue-151454.BQQ7up.rst
@@ -0,0 +1,7 @@
+When called without extra parameters, :meth:`email.message.Message.add_header`
+now stores the value the same way ``msg[name] = value`` does, so it accepts the
+:class:`~email.header.Header` instances that :meth:`~email.message.Message.items`
+returns under the ``compat32`` policy.  Previously it raised ``TypeError:
+sequence item 0: expected str instance, Header found``.  Combining a
+:class:`~email.header.Header` value with parameters now raises a clear
+``TypeError`` instead of that same confusing message.