diff --git a/tests/test_toml_document.py b/tests/test_toml_document.py
index f5695f6..59b42d7 100644
--- a/tests/test_toml_document.py
+++ b/tests/test_toml_document.py
@@ -669,3 +669,16 @@ def test_repr():
     )
 
     assert repr(doc["namespace"]) == "{'key1': 'value1', 'key2': 'value2'}"
+
+
+def test_deepcopy():
+    content = """
+[tool]
+name = "foo"
+[tool.project.section]
+option = "test"
+"""
+    doc = parse(content)
+    copied = copy.deepcopy(doc)
+    assert copied == doc
+    assert copied.as_string() == content
diff --git a/tomlkit/container.py b/tomlkit/container.py
index 4e3ea12..e097636 100644
--- a/tomlkit/container.py
+++ b/tomlkit/container.py
@@ -654,13 +654,18 @@ def __reduce_ex__(self, protocol):
         return (
             self.__class__,
             self._getstate(protocol),
-            (self._map, self._body, self._parsed),
+            (self._map, self._body, self._parsed, self._table_keys),
         )
 
     def __setstate__(self, state):
         self._map = state[0]
         self._body = state[1]
         self._parsed = state[2]
+        self._table_keys = state[3]
+
+        for key, item in self._body:
+            if key is not None:
+                dict.__setitem__(self, key.key, item.value)
 
     def copy(self):  # type: () -> Container
         return copy.copy(self)