diff --git a/autoload/fugitive.vim b/autoload/fugitive.vim
index b0855b4..d8929b3 100644
--- a/autoload/fugitive.vim
+++ b/autoload/fugitive.vim
@@ -1316,6 +1316,12 @@ function! fugitive#getfperm(url) abort
   return perm ==# '---------' ? '' : perm
 endfunction
 
+function s:UpdateIndex(dir, info) abort
+  let info = join(a:info[0:-2]) . "\t" . a:info[-1] . "\n"
+  let [error, exec_error] = s:SystemError([a:dir, 'update-index', '--index-info'], info)
+  return !exec_error ? '' : len(error) ? error : 'fugitive: unknown update-index error'
+endfunction
+
 function! fugitive#setfperm(url, perm) abort
   let [dir, commit, file] = s:DirCommitFile(a:url)
   let entry = s:PathInfo(a:url)
@@ -1324,9 +1330,8 @@ function! fugitive#setfperm(url, perm) abort
       \ substitute(perm, 'x', '-', 'g') !=# substitute(a:perm, 'x', '-', 'g')
     return -2
   endif
-  let exec_error = s:SystemError([dir, 'update-index', '--index-info'],
-        \ (a:perm =~# 'x' ? '000755 ' : '000644 ') . entry[3] . ' ' . commit . "\t" . file[1:-1])[1]
-  return exec_error ? -1 : 0
+  let error = s:UpdateIndex(dir, [a:perm =~# 'x' ? '000755' : '000644', entry[3], commit, file[1:-1]])
+  return len(error) ? -1 : 0
 endfunction
 
 function! s:TempCmd(out, cmd) abort
@@ -1396,9 +1401,8 @@ function! fugitive#writefile(lines, url, ...) abort
     let [hash, exec_error] = s:ChompError([dir, 'hash-object', '-w', temp])
     let mode = len(entry[1]) ? entry[1] : '100644'
     if !exec_error && hash =~# '^\x\{40,\}$'
-      let exec_error = s:SystemError([dir, 'update-index', '--index-info'],
-            \ mode . ' ' . hash . ' ' . commit . "\t" . file[1:-1])[1]
-      if !exec_error
+      let error = s:UpdateIndex(dir, [mode, hash, commit, file[1:-1]])
+      if empty(error)
         return 0
       endif
     endif
@@ -1451,9 +1455,8 @@ function! fugitive#delete(url, ...) abort
   if entry[2] !=# 'blob'
     return -1
   endif
-  let exec_error = s:SystemError([dir, 'update-index', '--index-info'],
-        \ '000000 0000000000000000000000000000000000000000 ' . commit . "\t" . file[1:-1])[1]
-  return exec_error ? -1 : 0
+  let error = s:UpdateIndex(dir, ['000000', '0000000000000000000000000000000000000000', commit, file[1:-1]])
+  return len(error) ? -1 : 0
 endfunction
 
 " Section: Buffer Object
@@ -2050,9 +2053,8 @@ function! fugitive#FileWriteCmd(...) abort
     if empty(old_mode)
       let old_mode = executable(s:Tree(dir) . file) ? '100755' : '100644'
     endif
-    let info = old_mode.' '.sha1.' '.commit."\t".file[1:-1]
-    let [error, exec_error] = s:SystemError([dir, 'update-index', '--index-info'], info . "\n")
-    if !exec_error
+    let error = s:UpdateIndex(dir, [old_mode, sha1, commit, file[1:-1]])
+    if empty(error)
       setlocal nomodified
       if exists('#' . autype . 'WritePost')
         execute s:DoAutocmd(autype . 'WritePost ' . s:fnameescape(amatch))