:silent won't suppress the press ENTER prompt

Hello, all

I have made a mapping to format the current buffer with
groff:

:nnoremap <F6> :silent! %!groff -s -rHm=1 -rHy=1 -U -t -p -mm -mant-tty -mru -mljinit -Tlatin1 -P-cbu<CR>

But it always end with the promt:

Press ENTER or type command to continue.

ignoring the :silent! command. Is it expected behavior?

--
Please, do not forward replies to the list to my e-mail.

--
--
You received this message from the "vim_use" maillist.
Do not top-post! Type your reply below the text you are replying to.
For more information, visit http://www.vim.org/maillist.php

---
You received this message because you are subscribed to the Google Groups "vim_use" group.
To unsubscribe from this group and stop receiving emails from it, send an email to vim_use+unsubscribe@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.