TypeError: s.concat is not a function

[rb200505]

Hi
I am getting the following error while using fields option as given in the example. Can you please help.
fields: {
edit: {
name: "edit-btn",
html: function (item) { return "Edit"; }
}
}
Error:
Uncaught (in promise) TypeError: s.concat is not a function
at t.value (kingtable.js:12)
at t.value (kingtable.js:14)
at t.value (kingtable.js:14)
at t.value (kingtable.js:14)
at t.value (kingtable.js:13)
at i (kingtable.js:13)
at kingtable.js:13

[rb200505]

and if I use as below, error is gone but the button is not displayed
fields: [
{
name: "delete-btn",
displayName: "Delete",
html: function () { "Delete"; }
}
],