diff --git a/lib/json0.js b/lib/json0.js
index 4c92407..4547e83 100644
--- a/lib/json0.js
+++ b/lib/json0.js
@@ -34,6 +34,8 @@ var isObject = function(obj) {
   return (!!obj) && (obj.constructor === Object);
 };
 
+var hasOwn = Object.hasOwn || Object.prototype.hasOwnProperty.call;
+
 /**
  * Clones the passed object using JSON serialization (which is slow).
  *
@@ -57,7 +59,7 @@ var json = {
 // You can register another OT type as a subtype in a JSON document using
 // the following function. This allows another type to handle certain
 // operations instead of the builtin JSON type.
-var subtypes = {};
+var subtypes = Object.create(null);
 json.registerSubtype = function(subtype) {
   subtypes[subtype.name] = subtype;
 };
@@ -177,6 +179,9 @@ json.apply = function(snapshot, op) {
     for (var j = 0; j < c.p.length; j++) {
       var p = c.p[j];
 
+      if (p in elem && !hasOwn(elem, p))
+        throw new Error('Path invalid');
+
       parent = elem;
       parentKey = key;
       elem = elem[key];
diff --git a/test/json0.coffee b/test/json0.coffee
index 05294e6..85cbf28 100644
--- a/test/json0.coffee
+++ b/test/json0.coffee
@@ -452,6 +452,16 @@ genTests = (type) ->
       assert.throws -> type.apply {x:'a'}, [{p:['x'], oi: 'c', od: 'b'}]
       assert.throws -> type.apply {x:'a'}, [{p:['x'], oi: 'b'}]
 
+    it 'disallows reassignment of special JS property names', ->
+      assert.throws -> type.apply {x:'a'}, [{p:['__proto__'], oi:'oops'}]
+      assert.throws -> type.apply {x:{y:'a'}}, [{p:['x', '__proto__'], oi:'oops'}]
+      assert.throws -> type.apply {x:'a'}, [{p:['constructor'], oi:'oops'}]
+      assert.throws -> type.apply {x:{y:'a'}}, [{p:['x', 'constructor'], oi:'oops'}]
+
+    it 'disallows modification of prototype property objects', ->
+      obj = {x:'a'}
+      assert.throws -> type.apply obj, [{p:['toString', 'name'], oi:'oops'}]
+
     it 'throws when the insertion key is a number', ->
       assert.throws -> type.apply {'1':'a'}, [{p:[2], oi: 'a'}]