diff --git a/tslib.es6.js b/tslib.es6.js
index c60aa60..bae9845 100644
--- a/tslib.es6.js
+++ b/tslib.es6.js
@@ -22,6 +22,8 @@ var extendStatics = function(d, b) {
 };
 
 export function __extends(d, b) {
+    if (typeof b !== "function" && b !== null)
+        throw new TypeError("Class extends value " + String(b) + " is not a constructor or null");
     extendStatics(d, b);
     function __() { this.constructor = d; }
     d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
diff --git a/tslib.js b/tslib.js
index fcf619a..b56c529 100644
--- a/tslib.js
+++ b/tslib.js
@@ -65,6 +65,8 @@ var __createBinding;
         function (d, b) { for (var p in b) if (Object.prototype.hasOwnProperty.call(b, p)) d[p] = b[p]; };
 
     __extends = function (d, b) {
+        if (typeof b !== "function" && b !== null)
+            throw new TypeError("Class extends value " + String(b) + " is not a constructor or null");
         extendStatics(d, b);
         function __() { this.constructor = d; }
         d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());