diff --git a/wp-admin/includes/post.php b/wp-admin/includes/post.php
index 74b3510163..1b2812c4a3 100644
--- a/wp-admin/includes/post.php
+++ b/wp-admin/includes/post.php
@@ -1380,11 +1380,12 @@ function _wp_post_thumbnail_html( $thumbnail_id = null, $post = null ) {
 
 	$post               = get_post( $post );
 	$post_type_object   = get_post_type_object( $post->post_type );
-	$set_thumbnail_link = '<p class="hide-if-no-js"><a href="%s" id="set-post-thumbnail" aria-describedby="set-post-thumbnail-desc" class="thickbox">%s</a></p>';
+	$set_thumbnail_link = '<p class="hide-if-no-js"><a href="%s" id="set-post-thumbnail"%s class="thickbox">%s</a></p>';
 	$upload_iframe_src  = get_upload_iframe_src( 'image', $post->ID );
 
 	$content = sprintf( $set_thumbnail_link,
 		esc_url( $upload_iframe_src ),
+		'', // Empty when there's no featured image set, `aria-describedby` attribute otherwise.
 		esc_html( $post_type_object->labels->set_featured_image )
 	);
 
@@ -1416,6 +1417,7 @@ function _wp_post_thumbnail_html( $thumbnail_id = null, $post = null ) {
 			$ajax_nonce = wp_create_nonce( 'set_post_thumbnail-' . $post->ID );
 			$content = sprintf( $set_thumbnail_link,
 				esc_url( $upload_iframe_src ),
+				' aria-describedby="set-post-thumbnail-desc"',
 				$thumbnail_html
 			);
 			$content .= '<p class="hide-if-no-js howto" id="set-post-thumbnail-desc">' . __( 'Click the image to edit or update' ) . '</p>';
diff --git a/wp-includes/version.php b/wp-includes/version.php
index e88ab58fe7..ab770dba19 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.5-alpha-36583';
+$wp_version = '4.5-alpha-36584';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.