diff --git a/stdlib/3/email/header.pyi b/stdlib/3/email/header.pyi
index 051e57536..f446d4a93 100644
--- a/stdlib/3/email/header.pyi
+++ b/stdlib/3/email/header.pyi
@@ -18,7 +18,7 @@ class Header:
     def __eq__(self, other: Any) -> bool: ...
     def __ne__(self, other: Any) -> bool: ...
 
-def decode_header(header: Header) -> List[Tuple[bytes, Optional[str]]]: ...
+def decode_header(header: Union[Header, str]) -> List[Tuple[bytes, Optional[str]]]: ...
 def make_header(decoded_seq: List[Tuple[bytes, Optional[str]]],
                 maxlinelen: Optional[int] =...,
                 header_name: Optional[str] = ...,