diff --git a/kphp_checks.php b/kphp_checks.php
new file mode 100644
index 0000000..2d1b5b5
--- /dev/null
+++ b/kphp_checks.php
@@ -0,0 +1,42 @@
+<?php
+
+#ifndef KPHP
+
+function typeof($arg)
+{
+	return get_class($arg);
+}
+
+
+function kphp_polyfill_getDeclaredProps($v, array &$errors):array
+{
+	$errors = [];
+	$refClass = new ReflectionClass($v);
+	$declaredProps = [];
+	foreach ( $refClass->getProperties(ReflectionProperty::IS_PUBLIC|ReflectionProperty::IS_PROTECTED|ReflectionProperty::IS_PRIVATE) as $prop ) {
+		$declaredProps[$prop->name] = 1;
+	};
+	return $declaredProps;
+}
+function kphp_polyfill_checkPropExists(?array &$declaredProps, string $propName, $value, array &$errors)
+{
+	if (!isset($declaredProps[$propName])) {
+		$type = gettype($value);
+		$errors [] = "public $type \$$propName = null;";
+	}
+}
+function kphp_polyfill_verifyProps($v, array $errors)
+{
+	if  (count($errors)) {
+		throw new Exception("Fields not declared as property in ".get_class($v) ."\n".implode("\n", $errors));
+	}
+}
+
+function kphp_polyfill_checkInstance(?object $instance, string $class_name)
+{
+	if  ($instance && ! ($instance instanceof $class_name)) {
+		throw new Exception("Can't cast instance of ".get_class($instance) ." to class $class_name.");
+	}
+}
+#endif
+
diff --git a/kphp_polyfills.php b/kphp_polyfills.php
index 9239b34..21d432d 100644
--- a/kphp_polyfills.php
+++ b/kphp_polyfills.php
@@ -152,6 +152,7 @@ function instance_cache_delete(string $key): bool {
  */
 function instance_cast($instance, string $class_name) {
   // it just returns the argument, but KPHP infers it as SomeClass
+  kphp_polyfill_checkInstance($instance, $class_name);
   return $instance;
 }
 
@@ -180,13 +181,22 @@ function to_array_debug($instance, $with_class_names = false, $public_members_on
 
   $toArray = function($v) use (&$toArray, &$demangleField, &$with_class_names, $public_members_only, $isPrivateField) {
     if (is_object($v)) {
+      if ($v instanceof \stdClass) {
+         throw new Exception("Invalid use stdclass");
+      }
       $result = [];
+      $errors = [];
+      $declaredProps = kphp_polyfill_getDeclaredProps($v, $errors);
       foreach ((array)$v as $field => $value) {
+         $propName = $demangleField($field);
         if ($public_members_only && $isPrivateField($field)) {
           continue;
         }
+         kphp_polyfill_checkPropExists($declaredProps, $propName, $value, $errors);
+
         $result[$demangleField($field)] = $toArray($value);
       }
+       kphp_polyfill_verifyProps($v, $errors);
       if ($with_class_names) {
         $result['__class_name'] = get_class($v);
       }
@@ -206,7 +216,7 @@ function to_array_debug($instance, $with_class_names = false, $public_members_on
  * @see to_array_debug function
  */
 function instance_to_array($instance, $with_class_names = false) {
-  return to_array_debug($instance, $with_class_names);
+  return to_array_debug($instance, $with_class_names, true);
 }
 
 #endregion
@@ -1124,3 +1134,5 @@ function ffi_array_get(\FFI\CData $arr, int $index) {
 
 
 #endif
+
+require_once __DIR__ .'/kphp_checks.php';