Code and tests

src/wp-includes/feed.php

@@ -837,8 +837,14 @@ function fetch_feed( $url ) {
 	 */
 	do_action_ref_array( 'wp_feed_options', array( &$feed, $url ) );
 
+	$blog_charset = get_bloginfo( 'charset' );
+
+	if ( empty( $blog_charset ) ) {
+		$blog_charset = 'UTF-8';
+	}
+
 	$feed->init();
-	$feed->set_output_encoding( get_option( 'blog_charset' ) );
+	$feed->set_output_encoding( $blog_charset );
 
 	if ( $feed->error() ) {
 		return new WP_Error( 'simplepie-error', $feed->error() );

tests/phpunit/tests/feed/fetchFeed.php

@@ -0,0 +1,45 @@
+<?php
+/**
+ * Tests the `fetch_feed` function.
+ *
+ * @package WordPress
+ * @subpackage UnitTests
+ * @since 6.7.0
+ *
+ * @group feed
+ */
+class Tests_Feed_fetchFeed extends WP_UnitTestCase {
+
+	public function set_up() {
+		parent::set_up();
+
+		add_filter( 'pre_http_request', array( $this, 'mocked_rss_response' ) );
+	}
+
+	/**
+	 * @ticket 62354
+	 */
+	public function test_empty_charset_does_not_trigger_fatal_error() {
+		add_filter( 'pre_option_blog_charset', '__return_empty_string', 20 );
+
+		$feed = fetch_feed( 'https://wordpress.org/news/feed/' );
+	}
+
+	public function mocked_rss_response() {
+		$single_value_headers = array(
+			'Content-Type' => 'application/rss+xml; charset=UTF-8',
+			'link'         => '<https://wordpress.org/news/wp-json/>; rel="https://api.w.org/"',
+		);
+
+		return array(
+			'headers'  => new WpOrg\Requests\Utility\CaseInsensitiveDictionary( $single_value_headers ),
+			'body'     => file_get_contents( DIR_TESTDATA . '/feed/wordpress-org-news.xml' ),
+			'response' => array(
+				'code'    => 200,
+				'message' => 'OK',
+			),
+			'cookies'  => array(),
+			'filename' => null,
+		);
+	}
+}