diff --git a/docs/cordis-catalog/services.md b/docs/cordis-catalog/services.md
index b9a0bc965a..36bbd7e3d4 100644
--- a/docs/cordis-catalog/services.md
+++ b/docs/cordis-catalog/services.md
@@ -398,7 +398,7 @@ abstract run(request: CodeRunRequest): Promise<CodeRunResult>
 
 Types: [CodeRunRequest](../core-data-structures/code-runtime.md) · [CodeRunResult](../core-data-structures/code-runtime.md)
 
-Source: [`packages/code-runtime/code-runtime/src/index.ts:104`](../../packages/code-runtime/code-runtime/src/index.ts)
+Source: [`packages/code-runtime/code-runtime/src/index.ts:105`](../../packages/code-runtime/code-runtime/src/index.ts)
 
 ## `ctx.commands` — `CommandService`
 
diff --git a/packages/code-runtime/code-runtime/src/index.ts b/packages/code-runtime/code-runtime/src/index.ts
index 3428b5c0e4..1f4c1ad287 100644
--- a/packages/code-runtime/code-runtime/src/index.ts
+++ b/packages/code-runtime/code-runtime/src/index.ts
@@ -59,10 +59,11 @@ export const RESERVED_ERROR_MEMBERS: ReadonlySet<string> = new Set([
 ])
 
 /**
- * Dunder form (`__x__`, non-empty middle): object-protocol slots in Python,
+ * Dunder form (`__x__`, distinct leading and trailing `__` pairs, so at least
+ * four characters; the middle may be empty): 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),