how to relock bootloader on my MOTO X 2014 XT1097?

I have a moto x 2014 xt1097 phone and i like to gift it to my girlfriend so i wanted to relock the boot loader and give it to her. but i am unable to relock the bootloader by the following cmd

fastboot oem lock

and i am getting some errors like
[ bootloader ] fastboot oem lock begin
[ bootloader ] flash signed images

can anyone suggest what is the proper procedure to relock the bootloader?

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