diff --git a/lib/internal/modules/esm/formats.js b/lib/internal/modules/esm/formats.js
index 470f679b92ec2a..ce606d95126d8c 100644
--- a/lib/internal/modules/esm/formats.js
+++ b/lib/internal/modules/esm/formats.js
@@ -26,7 +26,7 @@ if (experimentalWasmModules) {
 
 /**
  * @param {string} mime
- * @returns {string | null}
+ * @returns {string | undefined}
  */
 function mimeToFormat(mime) {
   if (
@@ -37,7 +37,7 @@ function mimeToFormat(mime) {
   ) { return 'module'; }
   if (mime === 'application/json') { return 'json'; }
   if (experimentalWasmModules && mime === 'application/wasm') { return 'wasm'; }
-  return null;
+  return undefined;
 }
 
 /**
diff --git a/lib/internal/modules/esm/get_format.js b/lib/internal/modules/esm/get_format.js
index cd5c88dce8e021..8a34bf48c12504 100644
--- a/lib/internal/modules/esm/get_format.js
+++ b/lib/internal/modules/esm/get_format.js
@@ -35,7 +35,7 @@ const protocolHandlers = {
 
 /**
  * @param {URL} parsed
- * @returns {string | null}
+ * @returns {string | undefined}
  */
 function getDataProtocolModuleFormat(parsed) {
   const { 1: mime } = RegExpPrototypeExec(
@@ -115,7 +115,7 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
         if (getOptionValue('--experimental-detect-module')) {
           const format = source ?
             (containsModuleSyntax(`${source}`, fileURLToPath(url), url) ? 'module' : 'commonjs') :
-            null;
+            undefined;
           if (format === 'module') {
             // This module has a .js extension, a package.json with no `type` field, and ESM syntax.
             // Warn about the missing `type` field so that the user can avoid the performance penalty of detection.
@@ -155,7 +155,7 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
       }
       default: { // The user did not pass `--experimental-default-type`.
         if (getOptionValue('--experimental-detect-module')) {
-          if (!source) { return null; }
+          if (!source) { return; }
           const format = getFormatOfExtensionlessFile(url);
           if (format === 'module') {
             return containsModuleSyntax(`${source}`, fileURLToPath(url), url) ? 'module' : 'commonjs';
@@ -201,7 +201,7 @@ function getHttpProtocolModuleFormat(url, context) {
 function defaultGetFormatWithoutErrors(url, context) {
   const protocol = url.protocol;
   if (!ObjectPrototypeHasOwnProperty(protocolHandlers, protocol)) {
-    return null;
+    return;
   }
   return protocolHandlers[protocol](url, context, true);
 }
@@ -214,7 +214,7 @@ function defaultGetFormatWithoutErrors(url, context) {
 function defaultGetFormat(url, context) {
   const protocol = url.protocol;
   if (!ObjectPrototypeHasOwnProperty(protocolHandlers, protocol)) {
-    return null;
+    return;
   }
   return protocolHandlers[protocol](url, context, false);
 }
diff --git a/test/es-module/test-esm-detect-ambiguous.mjs b/test/es-module/test-esm-detect-ambiguous.mjs
index 9d5f6f06a1c66b..106d59caf536d9 100644
--- a/test/es-module/test-esm-detect-ambiguous.mjs
+++ b/test/es-module/test-esm-detect-ambiguous.mjs
@@ -172,7 +172,7 @@ describe('--experimental-detect-module', { concurrency: !process.env.TEST_PARALL
       ]);
 
       strictEqual(stderr, '');
-      strictEqual(stdout, 'null\nexecuted\n');
+      strictEqual(stdout, 'undefined\nexecuted\n');
       strictEqual(code, 0);
       strictEqual(signal, null);