diff --git a/typescript/core/base.ts b/typescript/core/base.ts
index e681f2ac4..137f6c2da 100644
--- a/typescript/core/base.ts
+++ b/typescript/core/base.ts
@@ -178,7 +178,7 @@ export interface _base {
     functions: (object: object) => string[];
 
     extend<T extends object>(object: T): T;
-    extend<T extends object, U extends object>(object: T, ...sources: U[]): T & UnionToIntersection<U>;
+    extend<T extends object, U>(object: T, ...sources: U[]): T & UnionToIntersection<NonNullable<U>>;
 
     defaults: (object: object, ...sources: any[]) => object;