diff --git a/autoload/fugitive.vim b/autoload/fugitive.vim
index 2b05056..9bd089c 100644
--- a/autoload/fugitive.vim
+++ b/autoload/fugitive.vim
@@ -4633,6 +4633,8 @@ function! fugitive#Diffsplit(autodir, keepfocus, mods, arg, args) abort
       elseif arg =~# '^:\d$'
         exe s:DirCheck()
         let file = s:Relative(arg . ':')
+      elseif arg =~# '^[~^]\d*$'
+        return 'echoerr ' . string('fugitive: change ' . arg . ' to !' . arg . ' to diff against ancestor')
       else
         try
           let file = arg =~# '^:/.' ? fugitive#RevParse(arg) . s:Relative(':') : s:Expand(arg)