Transferring RESOURCE_ENERGY between creeps

[Waffle3z]

This documentation page http://support.screeps.com/hc/en-us/articles/203013212-Creep#transfer tells me that I can use Creep as an argument to the target parameter in creep.transfer, but it is giving me error -7 that "the target is not a valid object which can contain the specified resource." What is wrong?

edit: suddenly it is working.