diff --git a/stdlib/3/io.pyi b/stdlib/3/io.pyi
index 22a8784a7..ab32adc5f 100644
--- a/stdlib/3/io.pyi
+++ b/stdlib/3/io.pyi
@@ -206,4 +206,4 @@ class StringIO(TextIOWrapper):
     def __enter__(self) -> StringIO: ...
 
 class IncrementalNewlineDecoder(codecs.IncrementalDecoder):
-    def decode(self, input: bytes, final: bool = ...) -> str: ...
+    def decode(self, input: Union[bytes, str], final: bool = ...) -> str: ...