diff --git a/index.d.ts b/index.d.ts
index 0b72f17..dd9b55c 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -165,9 +165,9 @@ export const unit: {
 
 export function RBTreeWrapper(...args: any[]): any;
 
-export function UnifyError(...args: any[]): any;
+export class UnifyError extends Error {};
 
-export function NotAFunctionError(...args: any[]): any;
+export class NotAFunctionError extends Error {};
 
 export function addDouble(x: any): any;