Problem with "gpart add"

[Mike Clarke]

FreeBSD curlew.lan 9.1-RELEASE-p14 FreeBSD 9.1-RELEASE-p14 #0: Tue Jun  
3 08:56:12 UTC 2014     root at amd64-
builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64

I'm experiencing problems setting up a partition in some free space 
between two existing partitions on a drive:

This is the current setup:

curlew:/root# gpart show ada2
=>       63  625142385  ada2  MBR  (298G)
         63  125829081     1  freebsd  (60G)
  125829144  186731496        - free -  (89G)
  312560640  312576705     2  ntfs  (149G)
  625137345       5103        - free -  (2.5M)

I'd assumed that I could do this, but it fails

curlew:/root# gpart add -t freebsd-zfs -b 125829144 -s 186731496 ada2
gpart: Invalid argument

What am I doing wrong?

-- 
Mike Clarke