diff --git a/src/Reference/QrPaymentReferenceGenerator.php b/src/Reference/QrPaymentReferenceGenerator.php
index b875d75c..8a174c69 100644
--- a/src/Reference/QrPaymentReferenceGenerator.php
+++ b/src/Reference/QrPaymentReferenceGenerator.php
@@ -6,6 +6,7 @@
 use Sprain\SwissQrBill\Validator\SelfValidatableInterface;
 use Sprain\SwissQrBill\Validator\SelfValidatableTrait;
 use Symfony\Component\Validator\Constraints as Assert;
+use Symfony\Component\Validator\Context\ExecutionContextInterface;
 use Symfony\Component\Validator\Mapping\ClassMetadata;
 
 class QrPaymentReferenceGenerator implements SelfValidatableInterface
@@ -18,10 +19,13 @@ class QrPaymentReferenceGenerator implements SelfValidatableInterface
     /** @var string */
     private $referenceNumber;
 
-    public static function generate(string $customerIdentificationNumber, string $referenceNumber)
+    public static function generate(?string $customerIdentificationNumber, string $referenceNumber)
     {
         $qrPaymentReferenceGenerator = new self();
-        $qrPaymentReferenceGenerator->customerIdentificationNumber = $qrPaymentReferenceGenerator->removeWhitespace($customerIdentificationNumber);
+
+        if (null !== $customerIdentificationNumber) {
+            $qrPaymentReferenceGenerator->customerIdentificationNumber = $qrPaymentReferenceGenerator->removeWhitespace($customerIdentificationNumber);
+        }
         $qrPaymentReferenceGenerator->referenceNumber = $qrPaymentReferenceGenerator->removeWhitespace($referenceNumber);
 
         return $qrPaymentReferenceGenerator->doGenerate();
@@ -37,7 +41,7 @@ public function getReferenceNumber(): ?string
         return $this->referenceNumber;
     }
 
-    public function doGenerate()
+    private function doGenerate()
     {
         if (!$this->isValid()) {
             throw new InvalidQrPaymentReferenceException(
@@ -45,8 +49,8 @@ public function doGenerate()
             );
         }
 
-        $completeReferenceNumber  = str_pad($this->getCustomerIdentificationNumber(), 6, '0', STR_PAD_RIGHT);
-        $completeReferenceNumber .= str_pad($this->getReferenceNumber(), 20, '0', STR_PAD_LEFT);
+        $completeReferenceNumber  = $this->getCustomerIdentificationNumber();
+        $completeReferenceNumber .= str_pad($this->getReferenceNumber(), 26 - strlen($completeReferenceNumber), '0', STR_PAD_LEFT);
         $completeReferenceNumber .= $this->modulo10($completeReferenceNumber);
 
         return $completeReferenceNumber;
@@ -61,9 +65,8 @@ public static function loadValidatorMetadata(ClassMetadata $metadata): void
                 'match' => true
             ]),
             new Assert\Length([
-                'max' => 6
+                'max' => 11
             ]),
-            new Assert\NotBlank()
         ]);
 
         $metadata->addPropertyConstraints('referenceNumber', [
@@ -72,11 +75,18 @@ public static function loadValidatorMetadata(ClassMetadata $metadata): void
                 'pattern' => '/^\d*$/',
                 'match' => true
             ]),
-            new Assert\Length([
-                'max' => 20
-            ]),
             new Assert\NotBlank()
         ]);
+
+        $metadata->addConstraint(new Assert\Callback('validateFullReference'));
+    }
+
+    public function validateFullReference(ExecutionContextInterface $context, $payload)
+    {
+        if (strlen($this->customerIdentificationNumber) + strlen($this->referenceNumber) > 26) {
+            $context->buildViolation('The length of customer identification number + reference number may not exceed 26 characters in total.')
+                ->addViolation();
+        }
     }
 
     private function removeWhitespace(string $string): string
diff --git a/tests/Reference/QrPaymentReferenceGeneratorTest.php b/tests/Reference/QrPaymentReferenceGeneratorTest.php
index 1cb76eeb..178b93cd 100644
--- a/tests/Reference/QrPaymentReferenceGeneratorTest.php
+++ b/tests/Reference/QrPaymentReferenceGeneratorTest.php
@@ -40,6 +40,11 @@ public function qrPaymentReferenceProvider()
             ['040329', '340 ', '040329000000000000000003406'], // https://www.lukb.ch/documents/10620/13334/LUKB-BESR-Handbuch.pdf
             ['247656', '3073000002311006 ', '247656000030730000023110061'], // https://hilfe.flexbuero.ch/article/1181/
             ['123456', '11223344', '123456000000000000112233440'],
+            ['1234567890', '11223344', '123456789000000000112233444'],
+            ['1234', '11223344', '123400000000000000112233449'],
+            ['000000', '11223344', '000000000000000000112233442'],
+            ['', '11223344', '000000000000000000112233442'],
+            [null, '11223344', '000000000000000000112233442'],
 
             // Handle it as numerics as well
             [310014, 18310019779911119, '310014000183100197799111196'],
@@ -49,13 +54,35 @@ public function qrPaymentReferenceProvider()
         ];
     }
 
+    /**
+     * @dataProvider invalidQrPaymentReferenceProvider
+     * @expectedException Sprain\SwissQrBill\Validator\Exception\InvalidQrPaymentReferenceException
+     */
+    public function testInvalidQrPaymentReference($customerIdentification, $referenceNumber)
+    {
+        QrPaymentReferenceGenerator::generate(
+            $customerIdentification,
+            $referenceNumber
+        );
+    }
+
+    public function invalidQrPaymentReferenceProvider()
+    {
+        return [
+            ['1234', '12345678901234567890123'], // too long in total
+            ['123456', '123456789012345678901'], // too long in total
+            ['12345678901', '1234567890123456'], // too long in total
+            [null, '123456789012345678901234567'], // too long in total
+        ];
+    }
+
     /**
      * @dataProvider invalidCustomerIdentificationNumberProvider
      * @expectedException Sprain\SwissQrBill\Validator\Exception\InvalidQrPaymentReferenceException
      */
     public function testInvalidCustomerIdentificationNumber($value)
     {
-        $qrReference = QrPaymentReferenceGenerator::generate(
+        QrPaymentReferenceGenerator::generate(
             $value,
             '18310019779911119'
         );
@@ -64,10 +91,9 @@ public function testInvalidCustomerIdentificationNumber($value)
     public function invalidCustomerIdentificationNumberProvider()
     {
         return [
-            ['1234567'], // too long
+            ['123456789012'], // too long
             ['12345A'],  // non-digits
             ['1234.5'],  // non-digits
-            ['']
         ];
     }
 
@@ -77,7 +103,7 @@ public function invalidCustomerIdentificationNumberProvider()
      */
     public function testInvalidReferenceNumber($value)
     {
-        $qrReference = QrPaymentReferenceGenerator::generate(
+        QrPaymentReferenceGenerator::generate(
             '123456',
             $value
         );
@@ -86,7 +112,6 @@ public function testInvalidReferenceNumber($value)
     public function invalidReferenceNumberProvider()
     {
         return [
-            ['123456789012345678901'], // too long
             ['1234567890123456789A'],  // non-digits
             ['123456789012345678.0'],  // non-digits
             ['']