diff --git a/src/Assert.php b/src/Assert.php
index 31c861b9..bc9f3afd 100644
--- a/src/Assert.php
+++ b/src/Assert.php
@@ -406,9 +406,8 @@ public static function isInitialized(mixed $value, string $property, string|call
     {
         Assert::object($value);
 
-        $reflectionProperty = new ReflectionProperty($value, $property);
-
-        if (!$reflectionProperty->isInitialized($value)) {
+        if (!\property_exists($value, $property)
+            || !(new ReflectionProperty($value, $property))->isInitialized($value)) {
             $message = self::resolveMessage($message);
             static::reportInvalidArgument(\sprintf(
                 $message ?: 'Expected property %s to be initialized.',
diff --git a/tests/AssertTest.php b/tests/AssertTest.php
index f0ec064d..14ea6b9c 100644
--- a/tests/AssertTest.php
+++ b/tests/AssertTest.php
@@ -165,6 +165,10 @@ public static function getTests(): array
                 public mixed $a;
                 public mixed $b = true;
             }, 'b'], true],
+            ['isInitialized', [new class {
+                public mixed $b = true;
+            }, 'a'], false],
+            ['isInitialized', [new ArrayObject(), 'storage'], false],
             ['isCallable', ['strlen'], true],
             ['isCallable', [[self::class, 'getTests']], true],
             ['isCallable', [function () {}], true],