diff --git a/test/es-module/test-esm-export-not-found.mjs b/test/es-module/test-esm-export-not-found.mjs
index 1dc44e2bbac97a..0df42fba0532c7 100644
--- a/test/es-module/test-esm-export-not-found.mjs
+++ b/test/es-module/test-esm-export-not-found.mjs
@@ -25,7 +25,7 @@ const importStatementMultiline = `import {
   let stderr = '';
   child.stderr.setEncoding('utf8');
   child.stderr.on('data', (data) => {
-    stderr += data;
+    stderr += data.toString();
   });
   child.on('close', () => {
     // SyntaxError: The requested module './module-named-exports.mjs'
diff --git a/test/es-module/test-esm-import-json-named-export.mjs b/test/es-module/test-esm-import-json-named-export.mjs
index 73e8e9184972b4..649adea4a736af 100644
--- a/test/es-module/test-esm-import-json-named-export.mjs
+++ b/test/es-module/test-esm-import-json-named-export.mjs
@@ -12,7 +12,7 @@ const child = spawn(execPath, [
 let stderr = '';
 child.stderr.setEncoding('utf8');
 child.stderr.on('data', (data) => {
-  stderr += data;
+  stderr += data.toString();
 });
 child.on('close', () => {
   // SyntaxError: The requested module '../experimental.json'
diff --git a/test/es-module/test-esm-loader-not-found.mjs b/test/es-module/test-esm-loader-not-found.mjs
index 545127eb2412aa..e86e867696c99c 100644
--- a/test/es-module/test-esm-loader-not-found.mjs
+++ b/test/es-module/test-esm-loader-not-found.mjs
@@ -13,7 +13,7 @@ const child = spawn(execPath, [
 let stderr = '';
 child.stderr.setEncoding('utf8');
 child.stderr.on('data', (data) => {
-  stderr += data;
+  stderr += data.toString();
 });
 child.on('close', () => {
   // Error [ERR_MODULE_NOT_FOUND]: Cannot find package 'i-dont-exist'
diff --git a/test/es-module/test-esm-loader-obsolete-hooks.mjs b/test/es-module/test-esm-loader-obsolete-hooks.mjs
index a64456bdbe0a97..d297624d9d1981 100644
--- a/test/es-module/test-esm-loader-obsolete-hooks.mjs
+++ b/test/es-module/test-esm-loader-obsolete-hooks.mjs
@@ -15,7 +15,7 @@ const child = spawn(execPath, [
 let stderr = '';
 child.stderr.setEncoding('utf8');
 child.stderr.on('data', (data) => {
-  stderr += data;
+  stderr += data.toString();
 });
 child.on('close', () => {
   ok(stderr.includes(
diff --git a/test/es-module/test-esm-loader-with-syntax-error.mjs b/test/es-module/test-esm-loader-with-syntax-error.mjs
index d8ad0b5db27687..5305ae3b19be27 100644
--- a/test/es-module/test-esm-loader-with-syntax-error.mjs
+++ b/test/es-module/test-esm-loader-with-syntax-error.mjs
@@ -13,7 +13,7 @@ const child = spawn(execPath, [
 let stderr = '';
 child.stderr.setEncoding('utf8');
 child.stderr.on('data', (data) => {
-  stderr += data;
+  stderr += data.toString();
 });
 child.on('close', () => {
   ok(stderr.includes('SyntaxError:'));
diff --git a/test/es-module/test-esm-module-not-found-commonjs-hint.mjs b/test/es-module/test-esm-module-not-found-commonjs-hint.mjs
index ffe26b6980ffbd..a9452b67e6ec5a 100644
--- a/test/es-module/test-esm-module-not-found-commonjs-hint.mjs
+++ b/test/es-module/test-esm-module-not-found-commonjs-hint.mjs
@@ -26,7 +26,7 @@ import { execPath } from 'process';
   let stderr = '';
   child.stderr.setEncoding('utf8');
   child.stderr.on('data', (data) => {
-    stderr += data;
+    stderr += data.toString();
   });
   child.on('close', () => {
     ok(stderr.includes(expected));
diff --git a/test/es-module/test-esm-syntax-error.mjs b/test/es-module/test-esm-syntax-error.mjs
index a40aba09d845b5..b8635c176288fa 100644
--- a/test/es-module/test-esm-syntax-error.mjs
+++ b/test/es-module/test-esm-syntax-error.mjs
@@ -11,7 +11,7 @@ const child = spawn(execPath, [
 let stderr = '';
 child.stderr.setEncoding('utf8');
 child.stderr.on('data', (data) => {
-  stderr += data;
+  stderr += data.toString();
 });
 child.on('close', () => {
   ok(stderr.includes('SyntaxError:'));