diff --git a/typescript/core/base.ts b/typescript/core/base.ts
index 137f6c2da..5cc0a010c 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>(object: T, ...sources: U[]): T & UnionToIntersection<NonNullable<U>>;
+    extend<T extends object, U extends unknown[]>(object: T, ...sources: U): T & UnionToIntersection<NonNullable<U[number]>>;
 
     defaults: (object: object, ...sources: any[]) => object;