Automatic command to run at startup shows error of return value not being string type

I have the following in my tasks.json file:

{
        "label": "closealleditorsingroup",
        "type": "shell",
        "command": "${input:closealleditorsingroup}",
        "runOptions": {
            "runOn": "folderOpen"
        }
},

where, "inputs" is like so:

"inputs":[
    {
        "type": "command",
         "id": "closealleditorsingroup",
        "command": "workbench.action.closeEditorsInGroup"
    }
]

When I open a folder with folder>code ., VSCode starts up in that folder and runs this task automatically. All open editors from previous session are closed. Yet, I obtain the following error after:

Cannot substitute input variable 'closealleditorsingroup' because command 
'workbench.action.closeEditorsInGroup' did not return a result of type string.

Is there a way to fix this error?

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