diff --git a/lib/internal/modules/esm/get_format.js b/lib/internal/modules/esm/get_format.js
index 76b5ee6d147a49..f387b388274da6 100644
--- a/lib/internal/modules/esm/get_format.js
+++ b/lib/internal/modules/esm/get_format.js
@@ -91,12 +91,7 @@ function underNodeModules(url) {
  */
 function detectModuleFormat(source, url) {
   try {
-    let realSource = source ?? readFileSync(url, 'utf8');
-    if (Buffer.isBuffer(realSource)) {
-      // `containsModuleSyntax` requires source to be passed in as string
-      realSource = realSource.toString();
-    }
-    return containsModuleSyntax(realSource, fileURLToPath(url), url) ? 'module' : 'commonjs';
+    return containsModuleSyntax(source ?? readFileSync(url, 'utf8'), fileURLToPath(url), url) ? 'module' : 'commonjs';
   } catch {
     return 'commonjs';
   }