Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 51903)
+++ src/wp-includes/theme.php	(working copy)
@@ -1198,10 +1198,10 @@
  * @return string HTML image element markup or empty string on failure.
  */
 function get_header_image_tag( $attr = array() ) {
-	$header      = get_custom_header();
-	$header->url = get_header_image();
+	$header     = get_custom_header();
+	$header_url = get_header_image();
 
-	if ( ! $header->url ) {
+	if ( ! $header_url ) {
 		return '';
 	}
 
@@ -1211,7 +1211,7 @@
 	$attr = wp_parse_args(
 		$attr,
 		array(
-			'src'    => $header->url,
+			'src'    => $header_url,
 			'width'  => $width,
 			'height' => $height,
 			'alt'    => get_bloginfo( 'name' ),
@@ -1224,8 +1224,8 @@
 		$size_array = array( $width, $height );
 
 		if ( is_array( $image_meta ) ) {
-			$srcset = wp_calculate_image_srcset( $size_array, $header->url, $image_meta, $header->attachment_id );
-			$sizes  = ! empty( $attr['sizes'] ) ? $attr['sizes'] : wp_calculate_image_sizes( $size_array, $header->url, $image_meta, $header->attachment_id );
+			$srcset = wp_calculate_image_srcset( $size_array, $header_url, $image_meta, $header->attachment_id );
+			$sizes  = ! empty( $attr['sizes'] ) ? $attr['sizes'] : wp_calculate_image_sizes( $size_array, $header_url, $image_meta, $header->attachment_id );
 
 			if ( $srcset && $sizes ) {
 				$attr['srcset'] = $srcset;