diff --git a/Include/bytesobject.h b/Include/bytesobject.h
index b2d451a..ece06d2 100644
--- a/Include/bytesobject.h
+++ b/Include/bytesobject.h
@@ -91,8 +91,8 @@
    see Objects/stringlib/localeutil.h */
 
 PyAPI_FUNC(int) _PyBytes_InsertThousandsGrouping(char *buffer,
-						  Py_ssize_t len,
-						  char *plast,
+						  Py_ssize_t n_buffer,
+						  Py_ssize_t n_digits,
 						  Py_ssize_t buf_size,
 						  Py_ssize_t *count,
 						  int append_zero_char);