diff --git a/.changeset/fresh-apricots-heal.md b/.changeset/fresh-apricots-heal.md
new file mode 100644
index 0000000000..b85e9a5327
--- /dev/null
+++ b/.changeset/fresh-apricots-heal.md
@@ -0,0 +1,5 @@
+---
+"mobx": patch
+---
+
+Fix ES6 Map/Set checks for cross-window scripts
diff --git a/packages/mobx/src/types/observablemap.ts b/packages/mobx/src/types/observablemap.ts
index 99bd441750..3cf868b255 100644
--- a/packages/mobx/src/types/observablemap.ts
+++ b/packages/mobx/src/types/observablemap.ts
@@ -16,6 +16,7 @@ import {
     hasListeners,
     interceptChange,
     isES6Map,
+    isPlainES6Map,
     isPlainObject,
     isSpyEnabled,
     makeIterable,
@@ -351,7 +352,7 @@ export class ObservableMap<K = any, V = any>
             } else if (Array.isArray(other)) {
                 other.forEach(([key, value]) => this.set(key, value))
             } else if (isES6Map(other)) {
-                if (other.constructor !== Map) {
+                if (!isPlainES6Map(other)) {
                     die(19, other)
                 }
                 other.forEach((value, key) => this.set(key, value))
diff --git a/packages/mobx/src/utils/utils.ts b/packages/mobx/src/utils/utils.ts
index fc2760b04c..1f1ca701d7 100644
--- a/packages/mobx/src/utils/utils.ts
+++ b/packages/mobx/src/utils/utils.ts
@@ -141,11 +141,18 @@ export function createInstanceofPredicate<T>(
 }
 
 export function isES6Map(thing: any): thing is Map<any, any> {
-    return thing instanceof Map
+    return thing != null && Object.prototype.toString.call(thing) === "[object Map]"
+}
+
+export function isPlainES6Map(thing: Map<any, any>): boolean {
+    const mapProto = Object.getPrototypeOf(thing)
+    const objectProto = Object.getPrototypeOf(mapProto)
+    const nullProto = Object.getPrototypeOf(objectProto)
+    return nullProto === null
 }
 
 export function isES6Set(thing: any): thing is Set<any> {
-    return thing instanceof Set
+    return thing != null && Object.prototype.toString.call(thing) === "[object Set]"
 }
 
 const hasGetOwnPropertySymbols = typeof Object.getOwnPropertySymbols !== "undefined"