diff --git a/tslib.d.ts b/tslib.d.ts
index caed5d7..cc1927b 100644
--- a/tslib.d.ts
+++ b/tslib.d.ts
@@ -12,7 +12,7 @@ MERCHANTABLITY OR NON-INFRINGEMENT.
 See the Apache Version 2.0 License for specific language governing permissions
 and limitations under the License.
 ***************************************************************************** */
-export declare function __extends(d: Function, b: Function): void;
+export declare function __extends(d: Function, b: Function | null): void;
 export declare function __assign(t: any, ...sources: any[]): any;
 export declare function __rest(t: any, propertyNames: (string | symbol)[]): any;
 export declare function __decorate(decorators: Function[], target: any, key?: string | symbol, desc?: any): any;
diff --git a/tslib.es6.js b/tslib.es6.js
index 4d84182..b9fd081 100644
--- a/tslib.es6.js
+++ b/tslib.es6.js
@@ -22,9 +22,14 @@ var extendStatics = function(d, b) {
 };
 
 export function __extends(d, b) {
+    if (b === null) {
+        d.prototype = Object.create(b);
+        return;
+    }
     extendStatics(d, b);
     function __() { this.constructor = d; }
-    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+    __.prototype = b.prototype;
+    d.prototype = new __();
 }
 
 export var __assign = function() {
diff --git a/tslib.js b/tslib.js
index 459b59e..3e6add7 100644
--- a/tslib.js
+++ b/tslib.js
@@ -64,9 +64,14 @@ var __classPrivateFieldSet;
         function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
 
     __extends = function (d, b) {
+        if (b === null) {
+            d.prototype = Object.create(b);
+            return;
+        }
         extendStatics(d, b);
         function __() { this.constructor = d; }
-        d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+        __.prototype = b.prototype;
+        d.prototype = new __();
     };
 
     __assign = Object.assign || function (t) {