diff --git a/packages/bundle-source/src/is-entrypoint.js b/packages/bundle-source/src/is-entrypoint.js
index ce72b4ac26..58790eede1 100644
--- a/packages/bundle-source/src/is-entrypoint.js
+++ b/packages/bundle-source/src/is-entrypoint.js
@@ -3,7 +3,14 @@ import url from 'url';
 import process from 'process';
 import fs from 'fs';
 
+// Agoric still uses Endo dependencies under an emulation of ESM we call RESM
+// because it is invoked with `node -r esm`.
+// RESM does not support ?? nor ?. operators, so we must avoid them expressly.
+// TODO remove when https://github.com/Agoric/agoric-sdk/issues/8671
+const favor = (primary, secondary) =>
+  primary === undefined ? secondary : primary;
+
 // FIXME: Should maybe be exported by '@endo/something'?
 export const isEntrypoint = href =>
   String(href) ===
-  url.pathToFileURL(fs.realpathSync(process.argv[1]) ?? '/').href;
+  url.pathToFileURL(fs.realpathSync(favor(process.argv[1]), '/')).href;