Warning: array_merge() error in WP e-commerce

I am getting this error when I upload a photo in the WP e-commerce plugin. I am pretty sure it is related to the thumbnail options which are a little unclear.:
Warning: array_merge() [function.array-merge]: Argument #1 is not an array in /home/(mysite)/wp-content/plugins/wp-e-commerce/wpsc-admin/includes/display-items-functions.php on line 984

Here’s the corresponding template code:

// regenerate size metadata in case it's missing
		if ( ! image_get_intermediate_size( $post->ID, $check ) ) {
			if ( ! $metadata = wp_get_attachment_metadata( $post->ID ) )
				$metadata = array();
			$file = get_attached_file( $post->ID );
			$generated = wp_generate_attachment_metadata( $thumbnail_id, $file );
			$metadata['sizes'] = array_merge( $generated['sizes'], $metadata['sizes'] );
			wp_update_attachment_metadata( $post->ID, $metadata );
		}

Thanks!