diff --git a/src/IO/Validator/NotEmptyValidator.php b/src/IO/Validator/NotEmptyValidator.php
index eaafae86..d7bfc39f 100644
--- a/src/IO/Validator/NotEmptyValidator.php
+++ b/src/IO/Validator/NotEmptyValidator.php
@@ -41,7 +41,7 @@ final class NotEmptyValidator extends AbstractValidator
         'strict' => false,
     ];
 
-    public function __invoke(?string $input): ?string
+    public function __invoke(?string $input): string
     {
         if (null === $input || '' === $input) {
             throw Exception\ValidationException::create('The given input must not be empty.');
diff --git a/tests/src/Fixtures/ModifyingValidator.php b/tests/src/Fixtures/ModifyingValidator.php
index 4a9bf6a2..918ee7c2 100644
--- a/tests/src/Fixtures/ModifyingValidator.php
+++ b/tests/src/Fixtures/ModifyingValidator.php
@@ -41,7 +41,7 @@ final class ModifyingValidator extends IO\Validator\AbstractValidator
 {
     private const TYPE = 'modifying';
 
-    public function __invoke(?string $input): ?string
+    public function __invoke(?string $input): string
     {
         return strtoupper((string) $input);
     }