diff --git a/src/testdir/test_writefile.vim b/src/testdir/test_writefile.vim
index f462d04..9703323b 100644
--- a/src/testdir/test_writefile.vim
+++ b/src/testdir/test_writefile.vim
@@ -93,3 +93,10 @@
   endif
   bwipe Xfile
 endfunc
+
+func Test_writefile_sync_arg()
+  " This doesn't check if fsync() works, only that the argument is accepted.
+  call writefile(['one'], 'Xtest', 's')
+  call writefile(['two'], 'Xtest', 'S')
+  call delete('Xtest')
+endfunc