*** ERROR: `ethaddr' not set (4 replies)

I wanted to change the IP address of my GoFlexHome unit, so I ran:-

fw_setenv ipaddr 192.168.1.23

at a Debian command prompt. On reboot and stopping U-Boot and trying tftp xxxxxx yyyyy, I got an error msg saying "*** ERROR: `ethaddr' not set".
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I did not change ethaddr so why is it not set and how do I go about setting it to its previous value?