Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 18751)
+++ wp-includes/post.php	(working copy)
@@ -5191,6 +5191,26 @@
 }
 
 /**
+ * Removes a post thumbnail.
+ *
+ * @since 3.3.0
+ *
+ * @param int|object $post Post ID or object where thumbnail should be removed from.
+ * @return bool True on success, false on failure.
+ */
+function delete_post_thumbnail( $post ) {
+	$post = get_post( $post );
+	if ( $post ) {
+		$thumbnail_id = get_post_meta( $post->ID, '_thumbnail_id', true );
+		if ( ! empty( $thumbnail_id ) ) {
+			return delete_post_meta( $post->ID, '_thumbnail_id' );
+		}
+		return true;
+	}
+	return false;
+}
+
+/**
  * Returns a link to a post format index.
  *
  * @since 3.1.0
Index: wp-admin/admin-ajax.php
===================================================================
--- wp-admin/admin-ajax.php	(revision 18751)
+++ wp-admin/admin-ajax.php	(working copy)
@@ -1473,8 +1473,10 @@
 	check_ajax_referer( "set_post_thumbnail-$post_ID" );
 
 	if ( $thumbnail_id == '-1' ) {
-		delete_post_meta( $post_ID, '_thumbnail_id' );
-		die( _wp_post_thumbnail_html() );
+		if ( delete_post_thumbnail( $post_ID ) )
+			die( _wp_post_thumbnail_html() );
+		else
+			die( '0' );
 	}
 
 	if ( set_post_thumbnail( $post_ID, $thumbnail_id ) )