An error of type E_ERROR was caused in line 503

I’ve had a poke around this forum and there are a couple of instances of this error, but with no fix, apart from a temporary one.

After I’ve created, edited and saved a Gallery which is then displayed using Responsive Lightbox, I get an email :

Error Details
=============
An error of type E_ERROR was caused in line 503 of the file /home/myaccount/public_html/mysite.co.uk/wp-content/plugins/responsive-lightbox/includes/class-frontend.php. Error message: Uncaught Error: Unsupported operand types in /home/myaccount/public_html/mysite.co.uk/wp-content/plugins/responsive-lightbox/includes/class-frontend.php:503
Stack trace:
#0 /home/myaccount/public_html/mysite.co.uk/wp-content/plugins/responsive-lightbox/includes/class-galleries.php(3840): Responsive_Lightbox_Frontend->get_unique_fields(Array, NULL)
#1 /home/myaccount/public_html/mysite.co.uk/wp-content/plugins/responsive-lightbox/includes/class-galleries.php(4187): Responsive_Lightbox_Galleries->save_gallery(Array, 2481, true)
#2 /home/myaccount/public_html/mysite.co.uk/wp-includes/class-wp-hook.php(307): Responsive_Lightbox_Galleries->save_revision(2481)
#3 /home/myaccount/public_html/mysite.co.uk/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters('', Array)
#4 /home/myaccount/public_html/mysite.co.uk/wp-includes/plugin.php(474): WP_Hook->do_action(Array)
#5 /home/myaccount/public_html/mysite.co.uk/wp-includes/r

The site appears to work fine, just that I get this message every time I edit a gallery (the site gets regular batches of new pictures)

The error is: Unsupported operand types in …/responsive-lightbox/includes/class-frontend.php

The bad line is:
return $defaults + $fields;

I don’t know php well enough to spot any error here

Anyone have an idea / are the developers aware of this?