diff --git a/src/wp-admin/custom-header.php b/src/wp-admin/custom-header.php
index 0955420961..1a8fc32311 100644
--- a/src/wp-admin/custom-header.php
+++ b/src/wp-admin/custom-header.php
@@ -977,7 +977,7 @@ wp_nonce_field( 'custom-header-options', '_wpnonce-custom-header-options' ); ?>
 	 * @param mixed $choice Which header image to select. Allows for values of 'random-default-image',
 	 * 	for randomly cycling among the default images; 'random-uploaded-image', for randomly cycling
 	 * 	among the uploaded images; the key of a default image registered for that theme; and
-	 * 	the key of an image uploaded for that theme (the basename of the URL).
+	 * 	the key of an image uploaded for that theme (the attachment ID of the image).
 	 *  Or an array of arguments: attachment_id, url, width, height. All are required.
 	 */
 	final public function set_header_image( $choice ) {
diff --git a/src/wp-includes/theme.php b/src/wp-includes/theme.php
index 43334865c2..1727a17711 100644
--- a/src/wp-includes/theme.php
+++ b/src/wp-includes/theme.php
@@ -1207,7 +1207,7 @@ function get_uploaded_header_images() {
 	foreach ( (array) $headers as $header ) {
 		$url = esc_url_raw( wp_get_attachment_url( $header->ID ) );
 		$header_data = wp_get_attachment_metadata( $header->ID );
-		$header_index = basename($url);
+		$header_index = $header->ID;
 
 		$header_images[$header_index] = array();
 		$header_images[$header_index]['attachment_id'] = $header->ID;