Hi Anh,
Thank you for your reply. I haven't included the meta-box plugin itself in my theme--I'm just using the plugin downloaded from your site. In my theme, I have standard code like "new RW_Meta_Box( $meta_box )".
Also, I was able to narrow the problem down to something changed in version 4.4.2 of the plugin. 4.4.1 works fine but 4.4.2 gives me the error.
Thanks!
Ira