Array function errors

I’m getting these messages:

Warning: array_merge() [function.array-merge]: Argument #2 is not an array in /home/content/53/10102353/html/wp-content/plugins/ambrosite-nextprevious-post-link-plus/ambrosite-post-link-plus.php on line 114

Warning: array_unique() expects parameter 1 to be array, null given in /home/content/53/10102353/html/wp-content/plugins/ambrosite-nextprevious-post-link-plus/ambrosite-post-link-plus.php on line 114

Using this code:

<?php previous_post_link_plus( array(
	'order_by' => 'custom',
	'meta_key' => 'EventStartDate',
	'in_same_tax' => 'public-programs',
	'format' => '%link &mdash;'
) ); ?>

I can’t figure out what’s wrong. If I comment out the first three array elements, leaving only the ‘format’ pair, it works, but the reason I got this plugin was to exclude a taxonomy in the next/previous links.

http://wordpress.org/plugins/ambrosite-nextprevious-post-link-plus/