Throwing array error.

gistfile1.php

function spine_excerpt_style_classes( $classes ) {
	global $post;
	if ( !is_singular() ) {
			
			if ( $post->post_excerpt ) {
				$classes[] = "summary-excerpted";
			} elseif ( strstr( $post->post_content, '<!--more-->' ) ) {
				$classes[] = "summary-divided";
			} elseif ( 'excerpt' === spine_get_option( 'archive_content_display' ) ) {
				$classes[] = "summary-truncated";
			} else {
				$classes[] = "summary-unabridged";
			}
			
			return $classes;
		
		}
	}
	
add_filter( 'post_class', 'spine_excerpt_style_classes' );