diff --git a/tests/FILEFORMAT b/tests/FILEFORMAT
index 3e511555a..c15e26553 100644
--- a/tests/FILEFORMAT
+++ b/tests/FILEFORMAT
@@ -25,7 +25,7 @@ if the data is sent but this is what should be checked afterwards. If
 before comparing with the one actually received by the client
 </datacheck>
 <size>
-number to return on a ftp SIZE command
+number to return on a ftp SIZE command (set to -1 to make this command fail)
 </size>
 <cmd>
 special purpose server-command to control its behavior *before* the