BlockInstanceName not working

Scripting
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1

Learning to use RhinoCommon, I’m trying to use Rhino.RunTime.TextFields.BlockInstanceName(). Haven’t found a way to make it work. This is the simplest version of what I’m trying to get:

import Rhino

getCommand = Rhino.Input.Custom.GetObject()
getCommand.SetCommandPrompt("Select block instances")
getCommand.GeometryFilter = Rhino.DocObjects.ObjectType.InstanceReference
getCommand.SubObjectSelect = False
get_rc = getCommand.GetMultiple(1, 0)

if __name__ == "__main__":
    if getCommand.CommandResult()==Rhino.Commands.Result.Success: # If user doesn't cancel command
        for i in range(getCommand.ObjectCount):
            obj = getCommand.Object(i).Object()
            block_name = Rhino.Runtime.TextFields.BlockInstanceName(str(obj.Id))
            print block_name
    else:
        print "No block instances selected."

Returns: “####”

RhinoCommon documentation says I should be using “BlockName” method instead, but I don’t see it in the module. Am I doing something wrong?