diff --git a/ModelDescriber/Annotations/SymfonyConstraintAnnotationReader.php b/ModelDescriber/Annotations/SymfonyConstraintAnnotationReader.php
index 311bebe..a6d3daa 100644
--- a/ModelDescriber/Annotations/SymfonyConstraintAnnotationReader.php
+++ b/ModelDescriber/Annotations/SymfonyConstraintAnnotationReader.php
@@ -48,7 +48,8 @@ class SymfonyConstraintAnnotationReader
 
         foreach ($annotations as $annotation) {
             if ($annotation instanceof Assert\NotBlank || $annotation instanceof Assert\NotNull) {
-                if ($annotation instanceof Assert\NotBlank && $annotation->allowNull) {
+                // To support symfony/validator < 4.3
+                if ($annotation instanceof Assert\NotBlank && \method_exists($annotation, 'allowNull') && $annotation->allowNull) {
                     // The field is optional
                     continue;
                 }
diff --git a/Tests/ModelDescriber/Annotations/SymfonyConstraintAnnotationReaderTest.php b/Tests/ModelDescriber/Annotations/SymfonyConstraintAnnotationReaderTest.php
index c781db1..36c5b78 100644
--- a/Tests/ModelDescriber/Annotations/SymfonyConstraintAnnotationReaderTest.php
+++ b/Tests/ModelDescriber/Annotations/SymfonyConstraintAnnotationReaderTest.php
@@ -49,6 +49,10 @@ class SymfonyConstraintAnnotationReaderTest extends TestCase
 
     public function testOptionalProperty()
     {
+        if (!\method_exists(Assert\NotBlank::class, 'allowNull')) {
+            $this->markTestSkipped('NotBlank::allowNull was added in symfony/validator 4.3.');
+        }
+
         $entity = new class() {
             /**
              * @Assert\NotBlank(allowNull = true)