Using newline character as part of mask attack

I have a MD5 hash which is computed as follows.

hash = md5(username\npassword);

As you see there is a new line character (\n) between the username and password. I know the user name and trying to recover the password. Here is the hashcat syntax I am trying with a known username and known three character password without success. 

.\hashcat64 -a 3 -m 0 --hex-charset -1  0a hash.txt username?1?a?a?a -O

What am I doing wrong here?