diff --git a/src/index.ts b/src/index.ts
index 79cf695..7c53a9f 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -179,8 +179,10 @@ export class Sakota<T extends object> implements ProxyHandler<T> {
   /**
    * Proxy handler trap for `Reflect.ownKeys()`.
    */
-  public ownKeys(obj: any): (string  | symbol)[] {
-    const keys = Reflect.ownKeys(obj);
+  public ownKeys(obj: any): (string | symbol)[] {
+    // FIXME: need to figure out why this return (string | number | symbol)[]
+    // which is not the same as the return type of ES2015 Reflect.ownKeys.
+    const keys = Reflect.ownKeys(obj) as (string | symbol)[];
     if (this.diff) {
       for (const key in this.diff.$set) {
         if (!Object.prototype.hasOwnProperty.call(this.diff.$set, key)) {