Critical Error – Filter by Brand Widget with child brands

As brands is a hierarchical taxonomy my client has created “sub” or “child” parents. When using the filter by brands widget, if a top-level brand is selected it all works fine. But if a child brand is selected then there is a fatal error:

Recoverable fatal error: Object of class WP_Error could not be converted to string in /var/www/vhosts/staging.the-fireplace.co.uk/httpdocs/wp/wp-includes/formatting.php on line 1117

We’ve got round this on the live site by only showing top-level brands, but would like to also show the child brands too.