diff --git a/mypy/stubdoc.py b/mypy/stubdoc.py
index 9612a09abb708..dbf47508bcf0f 100644
--- a/mypy/stubdoc.py
+++ b/mypy/stubdoc.py
@@ -37,7 +37,7 @@ class ArgSig:
     """Signature info for a single argument."""
 
     def __init__(
-        self, name: str, type: str | None = None, default: bool = False, default_value: str = "..."
+        self, name: str, type: str | None = None, *, default: bool = False, default_value: str = "..."
     ) -> None:
         self.name = name
         self.type = type