diff --git a/lib/eslint/flat-eslint.js b/lib/eslint/flat-eslint.js
index 8444153076d..458509567b4 100644
--- a/lib/eslint/flat-eslint.js
+++ b/lib/eslint/flat-eslint.js
@@ -261,7 +261,7 @@ function compareResultsByFilePath(a, b) {
  * Searches from the current working directory up until finding the
  * given flat config filename.
  * @param {string} cwd The current working directory to search from.
- * @returns {Promise<string|null>} The filename if found or `null` if not.
+ * @returns {Promise<string|undefined>} The filename if found or `undefined` if not.
  */
 function findFlatConfigFile(cwd) {
     return findUp(
@@ -326,7 +326,7 @@ async function loadFlatConfigFile(filePath) {
  * as override config file is not explicitly set to `false`, it will search
  * upwards from the cwd for a file named `eslint.config.js`.
  * @param {import("./eslint").ESLintOptions} options The ESLint instance options.
- * @returns {{configFilePath:string,basePath:string,error:Error|null}} Location information for
+ * @returns {{configFilePath:string|undefined,basePath:string,error:Error|null}} Location information for
  *      the config file.
  */
 async function locateConfigFileToUse({ configFile, cwd }) {