diff --git a/knockout.validation/knockout.validation.d.ts b/knockout.validation/knockout.validation.d.ts
index 8317f7552..3fb49abb9 100644
--- a/knockout.validation/knockout.validation.d.ts
+++ b/knockout.validation/knockout.validation.d.ts
@@ -142,7 +142,7 @@ interface KnockoutSubscribableFunctions<T> {
     isValid: KnockoutComputed<boolean>;
     isValidating: KnockoutObservable<boolean>;
     rules: KnockoutObservableArray<KnockoutValidationRule>;
-    isModified(val: boolean): void;
+    isModified: KnockoutObservable<boolean>;
 }
 
 declare module "knockout.validation" {