diff --git a/third_party/2and3/google/protobuf/message.pyi b/third_party/2and3/google/protobuf/message.pyi
index c00c878fb..2265e0356 100644
--- a/third_party/2and3/google/protobuf/message.pyi
+++ b/third_party/2and3/google/protobuf/message.pyi
@@ -1,4 +1,6 @@
-from typing import Any, Sequence, Optional, Tuple, Type, TypeVar
+import sys
+
+from typing import Any, Sequence, Optional, Tuple, Type, TypeVar, Union
 
 from .descriptor import (
     DescriptorBase,
@@ -15,6 +17,11 @@ class _ExtensionDict:
 
 _T = TypeVar("_T")
 
+if sys.version_info < (3,):
+    _Serialized = Union[bytes, buffer, unicode]
+else:
+    _Serialized = bytes
+
 class Message:
     DESCRIPTOR: Any
     def __deepcopy__(self, memo=...): ...
@@ -25,8 +32,8 @@ class Message:
     def Clear(self) -> None: ...
     def SetInParent(self) -> None: ...
     def IsInitialized(self) -> bool: ...
-    def MergeFromString(self, serialized: Any) -> int: ...  # TODO: we need to be able to call buffer() on serialized
-    def ParseFromString(self, serialized: Any) -> None: ...
+    def MergeFromString(self, serialized: _Serialized) -> int: ...
+    def ParseFromString(self, serialized: _Serialized) -> None: ...
     def SerializeToString(self, deterministic: bool = ...) -> bytes: ...
     def SerializePartialToString(self, deterministic: bool = ...) -> bytes: ...
     def ListFields(self) -> Sequence[Tuple[FieldDescriptor, Any]]: ...