Understanding unique keys for array children in React.js

I'm building a React component that accepts a JSON data source and creates a sortable table.

Each of the dynamic data rows has a unique key assigned to it but I'm still getting an error of:

Each child in an array should have a unique "key" prop.

Check the render method of TableComponent.

My TableComponent render method returns:

<table>
  <thead key="thead">
    <TableHeader columns={columnNames}/>
  </thead>
  <tbody key="tbody">
    { rows }
  </tbody>
</table>

The TableHeader component is a single row and also has a unique key assigned to it.

Each row in rows is built from a component with a unique key:

<TableRowItem key={item.id} data={item} columns={columnNames}/>

And the TableRowItem looks like this:

var TableRowItem = React.createClass({
  render: function() {

    var td = function() {
        return this.props.columns.map(function(c) {
          return <td key={this.props.data[c]}>{this.props.data[c]}</td>;
        }, this);
      }.bind(this);

    return (
      <tr>{ td(this.props.item) }</tr>
    )
  }
});

What is causing the unique key prop error?