Problem with E-Newsletter Plugin

I’m getting the following warning message that appears at the top of my e-newsletter plugin dashboard and members screen.

The references to the respective php files seem to be about a problem with e-newsletter members.

But, I have no problem creating new members.

Any ideas on what’s causing this problem and how to fix it?

Warning: Missing argument 2 for wpdb::prepare(), called in /home/lasmtea/public_html/wp-content/plugins/e-newsletter/email-newsletter-files/class.functions.php on line 588 and defined in /home/lasmtea/public_html/wp-includes/wp-db.php on line 990