Index: wp-admin/includes/post.php
===================================================================
--- wp-admin/includes/post.php	(revision 20464)
+++ wp-admin/includes/post.php	(working copy)
@@ -1145,8 +1145,9 @@
 	if ( empty( $post_id ) )
 		$post_id = $post_ID;

-	$set_thumbnail_link = '<p class="hide-if-no-js"><a title="' . esc_attr__( 'Set featured image' ) . '" href="' . esc_url( get_upload_iframe_src('image', $post_id) ) . '" id="set-post-thumbnail" class="thickbox">%s</a></p>';
-	$content = sprintf($set_thumbnail_link, esc_html__( 'Set featured image' ));
+	$upload_iframe_src = esc_url( get_upload_iframe_src('image', $post_id) );
+	$set_thumbnail_link = '<p class="hide-if-no-js"><a title="' . esc_attr__( 'Set featured image' ) . '" href="%s" id="set-post-thumbnail" class="thickbox">%s</a></p>';
+	$content = sprintf( $set_thumbnail_link, $upload_iframe_src, esc_html__( 'Set featured image' ) );

 	if ( $thumbnail_id && get_post( $thumbnail_id ) ) {
 		$old_content_width = $content_width;
@@ -1157,7 +1158,7 @@
 			$thumbnail_html = wp_get_attachment_image( $thumbnail_id, 'post-thumbnail' );
 		if ( !empty( $thumbnail_html ) ) {
 			$ajax_nonce = wp_create_nonce( "set_post_thumbnail-$post_id" );
-			$content = sprintf($set_thumbnail_link, $thumbnail_html);
+			$content = sprintf( $set_thumbnail_link, $upload_iframe_src, $thumbnail_html );
 			$content .= '<p class="hide-if-no-js"><a href="#" id="remove-post-thumbnail" onclick="WPRemoveThumbnail(\'' . $ajax_nonce . '\');return false;">' . esc_html__( 'Remove featured image' ) . '</a></p>';
 		}
 		$content_width = $old_content_width;