Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 26953)
+++ wp-includes/pluggable.php	(working copy)
@@ -1023,7 +1023,10 @@
 	$author  = get_userdata( $post->post_author );
 
 	// Who to notify? By default, just the post author, but others can be added.
-	$emails = array( $author->user_email );
+    $emails = array();
+    if ( ! empty( $author ) ) {
+        $emails[] = $author->user_email;
+    }
 
 	/**
 	 * Filter the list of emails to receive a comment notification.
@@ -1061,17 +1064,17 @@
 	$notify_author = apply_filters( 'comment_notification_notify_author', false, $comment_id );
 
 	// The comment was left by the author
-	if ( ! $notify_author && $comment->user_id == $post->post_author ) {
+	if ( ! $notify_author && $comment->user_id == $post->post_author && ! empty( $author ) ) {
 		unset( $emails[ $author->user_email ] );
 	}
 
 	// The author moderated a comment on their own post
-	if ( ! $notify_author && $post->post_author == get_current_user_id() ) {
+	if ( ! $notify_author && $post->post_author == get_current_user_id() && ! empty( $author ) ) {
 		unset( $emails[ $author->user_email ] );
 	}
 
 	// The post author is no longer a member of the blog
-	if ( ! $notify_author && ! user_can( $post->post_author, 'read_post', $post->ID ) ) {
+	if ( ! $notify_author && ! user_can( $post->post_author, 'read_post', $post->ID ) && ! empty( $author ) ) {
 		unset( $emails[ $author->user_email ] );
 	}