diff --git a/runtime/doc/undo.txt b/runtime/doc/undo.txt
index 6e2308f..1195a05 100644
--- a/runtime/doc/undo.txt
+++ b/runtime/doc/undo.txt
@@ -285,6 +285,13 @@
 	the undo file cannot be used, it would corrupt the text.  This also
 	happens when 'encoding' differs from when the undo file was written.
 *E825*  The undo file does not contain valid contents and cannot be used.
+*E826*  The undo file is encrypted but decryption failed.
+*E827*  The undo file is encrypted but this version of Vim does not support
+	encryption.  Open the file with another Vim.
+*E832*  The undo file is encrypted but 'key' is not set, the text file is not
+	encrypted.  This would happen if the text file was written by Vim
+	encrypted at first, and later overwritten by not encrypted text.
+	You probably want to delete this undo file.
 "Not reading undo file, owner differs"
 	The undo file is owned by someone else than the owner of the text
 	file.  For safety the undo file is not used.