diff --git a/types/bn.js/index.d.ts b/types/bn.js/index.d.ts
index 7aa239d205..48f20ba429 100644
--- a/types/bn.js/index.d.ts
+++ b/types/bn.js/index.d.ts
@@ -62,7 +62,7 @@ declare class BN {
     /**
      * @description returns true if the supplied object is a BN.js instance
      */
-    static isBN(b: any): boolean;
+    static isBN(b: any): b is BN;
 
     /**
      * @description  Convert number to red