diff --git a/implementations/javascript/packages/core/src/jsdictionary.ts b/implementations/javascript/packages/core/src/jsdictionary.ts
index 8b00da7..3aebfa7 100644
--- a/implementations/javascript/packages/core/src/jsdictionary.ts
+++ b/implementations/javascript/packages/core/src/jsdictionary.ts
@@ -1,5 +1,4 @@
-import { Equivalence, IsMap, _iterMap } from './flex';
-import { Dictionary, DictionaryType } from './dictionary';
+import { Equivalence, _iterMap } from './flex';
 
 export interface JsDictionary<V> {
     [key: string]: V;
@@ -7,9 +6,12 @@ export interface JsDictionary<V> {
 
 export namespace JsDictionary {
     export function isJsDictionary<V>(x: any): x is JsDictionary<V> {
-        // We accept only literal objects and objects created via `new Object`
-        // as dictionaries.
-        return Object.getPrototypeOf(Object.getPrototypeOf(x ?? false)) === null;
+        // We accept only literal objects and objects created via `new Object` as dictionaries.
+        // Furthermore, we require no function-valued `__as_preserve__` property to exist.
+        return typeof x === 'object'
+            && x !== null
+            && Object.getPrototypeOf(Object.getPrototypeOf(x)) === null
+            && typeof x.__as_preserve__ !== 'function';
     }
 
     export function from<V>(entries: Iterable<[symbol, V]>): JsDictionary<V> {