Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 21204)
+++ wp-includes/post.php	(working copy)
@@ -1180,7 +1180,7 @@
 		$default_capabilities = array_merge( $default_capabilities, $default_capabilities_for_mapping );
 	}
 
-	$capabilities = array_merge( $default_capabilities, $args->capabilities );
+	$capabilities = array_merge( $default_capabilities, (array)$args->capabilities );
 
 	// Remember meta capabilities for future reference.
 	if ( $args->map_meta_cap )