diff --git a/core/src/json/decoder.ts b/core/src/json/decoder.ts
index 0628bae..38d84b5 100644
--- a/core/src/json/decoder.ts
+++ b/core/src/json/decoder.ts
@@ -210,7 +210,8 @@ const objectMap = <A>(decoder: Decoder<A>): Decoder<ObjectMap<A>> =>
       return failure('expected object but found ' + typeof input);
     }
 
-    const result = {} as ObjectMap<A>;
+    // object without a prototype or built-in functions.
+    const result = Object.create(null) as ObjectMap<A>;
     for (const field in input) {
       // @ts-ignore
       const decoded = decoder.run(input[field]);
diff --git a/core/tests/suites/json.ts b/core/tests/suites/json.ts
index 3068767..7b2b107 100644
--- a/core/tests/suites/json.ts
+++ b/core/tests/suites/json.ts
@@ -59,7 +59,7 @@ const tests = group('json', [
       )),
     test('pair', () =>
       roundtrip(s.pair(s.string, s.number), ['wat', 2] as [string, number])),
-    test('tripple', () =>
+    test('triple', () =>
       roundtrip(s.triple(s.string, s.number, s.boolean), ['wat', 2, false] as [
         string,
         number,
@@ -72,7 +72,11 @@ const tests = group('json', [
           .dictionary(genString(), fc.boolean())
           .map((v) => new Map(Object.entries(v)))
       )),
-    test('json', () => roundtripTest(s.json, genJson())),
+    group('json', [
+      test('plain', () => roundtripTest(s.json, genJson())),
+      test('with "__proto__" key', () =>
+        roundtrip(s.json, [{ ['__proto__']: [] }])),
+    ]),
     group('recursive', [
       test('base case', () => {
         type List =
@@ -553,10 +557,17 @@ function genJson(lvl = 0): fc.Arbitrary<Json> {
   );
 }
 
-function roundtripTest<T>(schema: Schema<T>, gen: fc.Arbitrary<T>): void {
+function roundtripTest<T>(
+  schema: Schema<T>,
+  gen: fc.Arbitrary<T>,
+  assertOptions?: fc.Parameters<[v: T]>
+): void {
   fc.assert(
     fc.property(gen, (v) => roundtrip(schema, v)),
-    { includeErrorInReport: true }
+    {
+      includeErrorInReport: true,
+      ...assertOptions,
+    }
   );
 }