diff --git a/dist/cjs/reflect/reflect.js b/dist/cjs/reflect/reflect.js
--- a/dist/cjs/reflect/reflect.js
+++ b/dist/cjs/reflect/reflect.js
@@ -236,7 +236,19 @@ class ReflectMapImpl {
                 throw err;
             }
         }
-        this.obj[mapKeyToLocal(key)] = mapValueToLocal(this._field, value);
+        const localKey = mapKeyToLocal(key);
+        const localValue = mapValueToLocal(this._field, value);
+        if (localKey === "__proto__") {
+            Object.defineProperty(this.obj, localKey, {
+                value: localValue,
+                writable: true,
+                enumerable: true,
+                configurable: true,
+            });
+        }
+        else {
+            this.obj[localKey] = localValue;
+        }
         return this;
     }
     delete(key) {
diff --git a/dist/cjs/create.js b/dist/cjs/create.js
--- a/dist/cjs/create.js
+++ b/dist/cjs/create.js
@@ -138,6 +138,17 @@ function convertObjectValues(obj, fn) {
     const ret = {};
     for (const entry of Object.entries(obj)) {
-        ret[entry[0]] = fn(entry[1]);
+        const value = fn(entry[1]);
+        if (entry[0] === "__proto__") {
+            Object.defineProperty(ret, entry[0], {
+                value,
+                writable: true,
+                enumerable: true,
+                configurable: true,
+            });
+        }
+        else {
+            ret[entry[0]] = value;
+        }
     }
     return ret;
 }
diff --git a/dist/esm/create.js b/dist/esm/create.js
--- a/dist/esm/create.js
+++ b/dist/esm/create.js
@@ -135,6 +135,17 @@ function convertObjectValues(obj, fn) {
     const ret = {};
     for (const entry of Object.entries(obj)) {
-        ret[entry[0]] = fn(entry[1]);
+        const value = fn(entry[1]);
+        if (entry[0] === "__proto__") {
+            Object.defineProperty(ret, entry[0], {
+                value,
+                writable: true,
+                enumerable: true,
+                configurable: true,
+            });
+        }
+        else {
+            ret[entry[0]] = value;
+        }
     }
     return ret;
 }
diff --git a/dist/esm/reflect/reflect.js b/dist/esm/reflect/reflect.js
--- a/dist/esm/reflect/reflect.js
+++ b/dist/esm/reflect/reflect.js
@@ -229,7 +229,19 @@ class ReflectMapImpl {
                 throw err;
             }
         }
-        this.obj[mapKeyToLocal(key)] = mapValueToLocal(this._field, value);
+        const localKey = mapKeyToLocal(key);
+        const localValue = mapValueToLocal(this._field, value);
+        if (localKey === "__proto__") {
+            Object.defineProperty(this.obj, localKey, {
+                value: localValue,
+                writable: true,
+                enumerable: true,
+                configurable: true,
+            });
+        }
+        else {
+            this.obj[localKey] = localValue;
+        }
         return this;
     }
     delete(key) {