Problem geting IP address (doesn't look like a script error)

This works on routerboard
:global ifip [ /ip address get [/ip address find interface=WAN ] address ]
but not on x86 system

on a pc (and on one Soekris board with the x86 version installed on it) it drops out with error:
interrupted
invalid internal item number


Works fine on routerboard. The interface DOES exist and is correctly named (i c/p the name to be sure and still the error)

Version in question is 5rc1


Thanks