This is a buffer here

Author: GeoffreyBooth

lib/internal/modules/esm/get_format.js

@@ -127,7 +127,7 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
         // `source` is undefined when this is called from `defaultResolve`;
         // but this gets called again from `defaultLoad`/`defaultLoadSync`.
         if (getOptionValue('--experimental-detect-module')) {
-          const format = detectModuleFormat(source, url);
+          const format = detectModuleFormat(source?.toString(), url);
           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.