diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0af52c54..8b5fd0f9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 ### Fixed
 
+- Fix an `AttributeError` when a table is defined out of order under an indented header nested four or more levels deep, e.g. ` [a.b.c.d]` then `[a.b.c]`. ([#663](https://github.com/python-poetry/tomlkit/pull/663))
 - Fix valid TOML being rejected when an out-of-order table reopens an implicit super-table with a dotted key: `[a.b.c]` then `[a]` with `b.z = 1` extends the implicit `a.b` super-table rather than redefining it (stdlib `tomllib` accepts it), so it now parses and round-trips instead of raising `Redefinition of an existing table`. The concrete-table case (`[a.b]` then `[a]` `b.z = 1`) stays rejected. ([#565](https://github.com/python-poetry/tomlkit/pull/565))
 - Fix appending a key to a parsed inline table that uses `", "` separators emitting a bare `","` for the new entry, leaving the table inconsistently spaced (e.g. `{x = 1, y = 2,z = 3}`). ([#595](https://github.com/python-poetry/tomlkit/pull/595))
 
diff --git a/tests/test_toml_document.py b/tests/test_toml_document.py
index 84b114b9..09d1398a 100644
--- a/tests/test_toml_document.py
+++ b/tests/test_toml_document.py
@@ -675,6 +675,25 @@ def test_valid_out_of_order_super_table_extended_by_dotted_key() -> None:
     assert doc.as_string() == "[a.b.c]\n[a]\nb.z = 1\n"
 
 
+@pytest.mark.parametrize(
+    ("content", "expected"),
+    [
+        (" [a.b.c.d]\n[a.b.c]\n", {"a": {"b": {"c": {"d": {}}}}}),
+        (" [[a.b.c.d]]\n[a.b.c]\n", {"a": {"b": {"c": {"d": [{}]}}}}),
+        (" [a.b.c.d]\n[x]\n[a.b.c]\n", {"a": {"b": {"c": {"d": {}}}}, "x": {}}),
+    ],
+)
+def test_out_of_order_table_after_indented_header(
+    content: str, expected: dict[str, Any]
+) -> None:
+    # https://github.com/python-poetry/tomlkit/issues/263 covered `[a.b]` after
+    # an indented `[[a.b.c]]`. One level deeper, the out-of-order table lands
+    # under an intermediate table that carries the header's indent.
+    doc = parse(content)
+    assert doc.unwrap() == expected
+    assert doc.as_string() == content
+
+
 def test_set_value_on_out_of_order_table_with_empty_concrete_part() -> None:
     # A super table defined after its sub-table (the "defining a super-table
     # afterward is ok" spec example) leaves an empty concrete `[x]` part.
diff --git a/tomlkit/items.py b/tomlkit/items.py
index 26b01097..cc496c2e 100644
--- a/tomlkit/items.py
+++ b/tomlkit/items.py
@@ -1959,7 +1959,9 @@ def append(self, key: Key | str | None, _item: Any) -> Table:
 
         indent = m.group(1)
 
-        if not isinstance(_item, Whitespace):
+        # A table split out of order is looked up as an OutOfOrderTableProxy,
+        # which has no trivia; its parts keep their own indentation.
+        if isinstance(_item, Item) and not isinstance(_item, Whitespace):
             m = re.match("(?s)^([^ ]*)(.*)$", _item.trivia.indent)
             if not m:
                 _item.trivia.indent = indent