diff --git a/src/testdir/test_writefile.vim b/src/testdir/test_writefile.vim
index 5449071..497fa0b 100644
--- a/src/testdir/test_writefile.vim
+++ b/src/testdir/test_writefile.vim
@@ -16,6 +16,8 @@
   call assert_equal("morning", l[3])
   call assert_equal("vimmers", l[4])
   call delete(f)
+
+  call assert_fails('call writefile("text", "Xfile")', 'E475: Invalid argument: writefile() first argument must be a List or a Blob')
 endfunc
 
 func Test_writefile_ignore_regexp_error()