diff --git a/src/Formatter/NLFormatter.php b/src/Formatter/NLFormatter.php
index a764677..041bcef 100644
--- a/src/Formatter/NLFormatter.php
+++ b/src/Formatter/NLFormatter.php
@@ -19,7 +19,7 @@ final class NLFormatter implements CountryPostcodeFormatter
 {
     public function format(string $postcode) : ?string
     {
-        if (preg_match('/^([0-9]{4})([A-Z]{2})$/', $postcode, $matches) !== 1) {
+        if (preg_match('/^([1-9][0-9]{3})([A-Z]{2})$/', $postcode, $matches) !== 1) {
             return null;
         }
 
diff --git a/tests/Formatter/NLFormatterTest.php b/tests/Formatter/NLFormatterTest.php
index ad52725..b562b19 100644
--- a/tests/Formatter/NLFormatterTest.php
+++ b/tests/Formatter/NLFormatterTest.php
@@ -23,6 +23,7 @@ public function providerFormat() : array
         return [
             ['', null],
 
+            ['0123AB', null],
             ['1234AB', '1234 AB'],
             ['1234SS', null],
             ['1234SD', null],