diff --git a/python/pycrdt/_doc.py b/python/pycrdt/_doc.py
index 9658723..5ea08e2 100644
--- a/python/pycrdt/_doc.py
+++ b/python/pycrdt/_doc.py
@@ -36,6 +36,11 @@ def client_id(self) -> int:
 
     def transaction(self, origin: Any = None) -> Transaction:
         if self._txn is not None:
+            if origin is not None:
+                if origin != self._txn.origin:
+                    raise RuntimeError(
+                        "Nested transactions must have same origin as root transaction"
+                    )
             return self._txn
         return Transaction(self, origin=origin)
 
diff --git a/python/pycrdt/_transaction.py b/python/pycrdt/_transaction.py
index 9d54506..6128efa 100644
--- a/python/pycrdt/_transaction.py
+++ b/python/pycrdt/_transaction.py
@@ -13,22 +13,23 @@ class Transaction:
     _doc: Doc
     _txn: _Transaction | None
     _nb: int
+    _origin_hash: int | None
 
     def __init__(self, doc: Doc, _txn: _Transaction | None = None, *, origin: Any = None) -> None:
         self._doc = doc
         self._txn = _txn
         self._nb = 0
         if origin is None:
-            self._origin = None
+            self._origin_hash = None
         else:
-            self._origin = hash_origin(origin)
-            doc._origins[self._origin] = origin
+            self._origin_hash = hash_origin(origin)
+            doc._origins[self._origin_hash] = origin
 
     def __enter__(self) -> Transaction:
         self._nb += 1
         if self._txn is None:
-            if self._origin is not None:
-                self._txn = self._doc._doc.create_transaction_with_origin(self._origin)
+            if self._origin_hash is not None:
+                self._txn = self._doc._doc.create_transaction_with_origin(self._origin_hash)
             else:
                 self._txn = self._doc._doc.create_transaction()
         self._doc._txn = self
diff --git a/tests/test_transaction.py b/tests/test_transaction.py
index 93247df..3aff42f 100644
--- a/tests/test_transaction.py
+++ b/tests/test_transaction.py
@@ -89,6 +89,15 @@ def callback(event, txn):
     assert len(doc0._origins) == 0
     assert len(doc1._origins) == 0
 
+    with doc0.transaction(origin=123):
+        with doc0.transaction(origin=123):
+            with doc0.transaction():
+                with pytest.raises(RuntimeError) as excinfo:
+                    with doc0.transaction(origin=456):
+                        pass
+
+    assert str(excinfo.value) == "Nested transactions must have same origin as root transaction"
+
 
 def test_observe_callback_params():
     doc = Doc()