diff --git a/Doc/c-api/function.rst b/Doc/c-api/function.rst
index ada974c7656..3512fe2206a 100644
--- a/Doc/c-api/function.rst
+++ b/Doc/c-api/function.rst
@@ -93,3 +93,5 @@ There are a few functions specific to Python functions.
 
    Set the annotations for the function object *op*. *annotations*
    must be a dictionary or *Py_None*.
+
+   Raises :exc:`SystemError` and returns ``-1`` on failure.