diff --git a/wp-admin/admin-ajax.php b/wp-admin/admin-ajax.php
index 0e524ca24..2a4a2771f 100644
--- a/wp-admin/admin-ajax.php
+++ b/wp-admin/admin-ajax.php
@@ -1420,13 +1420,8 @@ case 'set-post-thumbnail':
 		die( _wp_post_thumbnail_html() );
 	}
 
-	if ( $thumbnail_id && get_post( $thumbnail_id ) ) {
-		$thumbnail_html = wp_get_attachment_image( $thumbnail_id, 'thumbnail' );
-		if ( !empty( $thumbnail_html ) ) {
-			update_post_meta( $post_ID, '_thumbnail_id', $thumbnail_id );
-			die( _wp_post_thumbnail_html( $thumbnail_id ) );
-		}
-	}
+	if ( set_post_thumbnail( $post_ID, $thumbnail_id ) )
+		die( _wp_post_thumbnail_html( $thumbnail_id ) );
 	die( '0' );
 	break;
 case 'date_format' :
diff --git a/wp-includes/post.php b/wp-includes/post.php
index 02c661d7e..189eae176 100644
--- a/wp-includes/post.php
+++ b/wp-includes/post.php
@@ -5257,4 +5257,25 @@ function get_post_format_string( $slug ) {
 	return ( isset( $strings[$slug] ) ) ? $strings[$slug] : '';
 }
 
+/**
+ * Sets a post thumbnail.
+ *
+ * @since 3.1.0
+ *
+ * @param int|object $post Post ID or object where thumbnail should be attached.
+ * @param int $thumbnail_id Thumbnail to attach.
+ * @return bool True on success, false on failure.
+ */
+function set_post_thumbnail( $post, $thumbnail_id ) {
+	$post = get_post( $post );
+	if ( $post && $thumbnail_id && get_post( $thumbnail_id ) ) {
+		$thumbnail_html = wp_get_attachment_image( $thumbnail_id, 'thumbnail' );
+		if ( ! empty( $thumbnail_html ) ) {
+			update_post_meta( $post->ID, '_thumbnail_id', $thumbnail_id );
+			return true;
+		}
+	}
+	return false;
+}
+
 ?>