Index: wp-includes/post-template.php
===================================================================
--- wp-includes/post-template.php	(revision 18462)
+++ wp-includes/post-template.php	(working copy)
@@ -433,12 +433,14 @@
 
 		// Post Format
 		$post_format = get_post_format( $post->ID );
+	
+		if ( post_type_supports( $post->post_type, 'post-formats' ) ) {
+			if ( $post_format && !is_wp_error($post_format) )
+				$classes[] = 'single-format-' . sanitize_html_class( $post_format );
+			else
+				$classes[] = 'single-format-standard';
+		}
 
-		if ( $post_format && !is_wp_error($post_format) )
-			$classes[] = 'single-format-' . sanitize_html_class( $post_format );
-		else
-			$classes[] = 'single-format-standard';
-
 		if ( is_attachment() ) {
 			$mime_type = get_post_mime_type($post_id);
 			$mime_prefix = array( 'application/', 'image/', 'text/', 'audio/', 'video/', 'music/' );