react: Removing tabIndex from component's attributes does not remove tabIndex once set

Once a tab index has been put in place “removing” by the method seen below only ever sets it to -1, which does what I’d expect in that the element isn’t focused when tabbing, but it is still focusable by clicking directly on the element.

var attributes = {}
if (this.props.focusable) {
  attributes.tabIndex = 0;
}

return React.createElement("div", attributes, "test");