diff --git a/source/lib/operators/not.ts b/source/lib/operators/not.ts
index aac1982..074db7a 100644
--- a/source/lib/operators/not.ts
+++ b/source/lib/operators/not.ts
@@ -6,7 +6,7 @@ import { Predicate, Validator, validatorSymbol } from '../predicates/predicate';
  * @param predictate Predicate to wrap inside the operator.
  */
 export const not = <T extends Predicate>(predicate: T) => {
-	predicate['addValidator'] = (validator: Validator<any>) => {		// tslint:disable-line:no-string-literal
+	predicate.addValidator = (validator: Validator<any>) => {
 		const fn = validator.validator;
 		const message = validator.message;
 
diff --git a/source/lib/predicates/predicate.ts b/source/lib/predicates/predicate.ts
index 030ae19..ad248d2 100644
--- a/source/lib/predicates/predicate.ts
+++ b/source/lib/predicates/predicate.ts
@@ -38,9 +38,10 @@ export abstract class Predicate<T = any> {
 	/**
 	 * Register a new validator.
 	 *
+	 * @internal
 	 * @param validator Validator to register.
 	 */
-	protected addValidator(validator: Validator<T>) {
+	addValidator(validator: Validator<T>) {
 		this.context.validators.push(validator);
 
 		return this;