diff --git a/packages/code-runtime/code-runtime/src/index.ts b/packages/code-runtime/code-runtime/src/index.ts
index 3555dbfa23..3428b5c0e4 100644
--- a/packages/code-runtime/code-runtime/src/index.ts
+++ b/packages/code-runtime/code-runtime/src/index.ts
@@ -59,10 +59,10 @@ export const RESERVED_ERROR_MEMBERS: ReadonlySet<string> = new Set([
 ])
 
 /**
- * Dunder form (`__…__`, two `__` pairs with an optionally empty middle): object-protocol
- * slots in Python, refused as {@link RESERVED_ERROR_MEMBERS | error members} on every backend.
+ * Dunder form (`__x__`, non-empty middle): object-protocol slots in Python,
+ * refused as {@link RESERVED_ERROR_MEMBERS | error members} on every backend.
  */
-export const DUNDER_MEMBER = /^__.*__$/
+export const DUNDER_MEMBER = /^__.+__$/
 
 /**
  * Reserved words of every portable target language (ECMAScript ∪ Python),