How to clear custom field value ?

[AnnaVinnikova]

Hi, I need auto rule to copy value from parent issue to child issue in the Components field. 
For now it saves both values in the child: the previous and the new one.
I tried to find how to clear the value before the updating

{
    "update" : {
        "TmpField" : [{"set":"" }]
    }
}

{ "fields" : {
        "TmpField" : null
    }
}


{ "fields" : {
        "TmpField" : ""
    }
}

I tried all of the options and it doesn't work

[Alex Koxaras _Relational_]

Hi @AnnaVinnikova 

What type of field is the one you are trying to copy?

[AnnaVinnikova]

Components

[Alex Koxaras _Relational_]

@AnnaVinnikova there isn't such a custom field type. The "TmpField" what type is it? Text? Single select? Labels? If you go to cog wheel (upper right corner) > issues > custom field and then search for TmpField, you will see the type of it on the second column of the results page.