diff --git a/src/types/observablemap.ts b/src/types/observablemap.ts
index 24f1e261a..12408040b 100644
--- a/src/types/observablemap.ts
+++ b/src/types/observablemap.ts
@@ -73,8 +73,8 @@ export type IObservableMapInitialValues<V> = IMapEntries<V> | IKeyValueMap<V> |
 
 export class ObservableMap<V> implements IInterceptable<IMapWillChange<V>>, IListenable, IMap<string, V> {
 	$mobx = ObservableMapMarker;
-	private _data: { [key: string]: ObservableValue<V | undefined> } = {};
-	private _hasMap: { [key: string]: ObservableValue<boolean> } = {}; // hasMap, not hashMap >-).
+	private _data: { [key: string]: ObservableValue<V | undefined> } = Object.create(null);
+	private _hasMap: { [key: string]: ObservableValue<boolean> } = Object.create(null); // hasMap, not hashMap >-).
 	private _keys: IObservableArray<string> = <any> new ObservableArray(undefined, referenceEnhancer, `${this.name}.keys()`, true);
 	interceptors = null;
 	changeListeners = null;
diff --git a/test/map.js b/test/map.js
index aa86c1a41..205bd8abf 100644
--- a/test/map.js
+++ b/test/map.js
@@ -603,3 +603,20 @@ test('using deep map', t => {
 
   t.end();
 });
+
+test("issue 893", t => {
+  const m = mobx.observable.map();
+  const keys = ['constructor', 'toString', 'assertValidKey', 'isValidKey', 'toJSON', 'toJS']
+  for (let key of keys) {
+	  t.equal(m.get(key), undefined);
+  }
+  t.end();
+});
+
+test("work with 'toString' key", t => {
+	const m = mobx.observable.map();
+	t.equal(m.get('toString'), undefined);
+	m.set('toString', 'test');
+	t.equal(m.get('toString'), 'test');
+	t.end();
+});
\ No newline at end of file