diff --git a/runtime/doc/editing.txt b/runtime/doc/editing.txt
index 32848ac..b4cdc48 100644
--- a/runtime/doc/editing.txt
+++ b/runtime/doc/editing.txt
@@ -1376,6 +1376,9 @@
 don't write the file encrypted!  You need to rebuild the Vim binary to fix
 this.
 
+*E831* This is an internal error, "cannot happen".  If you can reproduc it,
+please report to the developers.
+
 When reading a file that has been encrypted and the 'key' option is not empty,
 it will be used for decryption.  If the value is empty, you will be prompted
 to enter the key.  If you don't enter a key, or you enter the wrong key, the