diff --git a/stdlib/3/subprocess.pyi b/stdlib/3/subprocess.pyi
index f9e0f720e..c606c33db 100644
--- a/stdlib/3/subprocess.pyi
+++ b/stdlib/3/subprocess.pyi
@@ -29,7 +29,7 @@ class CalledProcessError(Exception):
     cmd = ...  # type: str
     output = b'' # May be None
 
-    def __init__(self, returncode: int, cmd: str, output: str) -> None: ...
+    def __init__(self, returncode: int, cmd: str, output: str = ...) -> None: ...
 
 class Popen:
     stdin = ... # type: IO[Any]