diff --git a/src/Inspector/Hash.php b/src/Inspector/Hash.php
index 8fe8c989..bf8fcc76 100644
--- a/src/Inspector/Hash.php
+++ b/src/Inspector/Hash.php
@@ -71,7 +71,7 @@ private function getValuesByType(TypeInterface $type): \Generator
     {
         if ($type instanceof StructDefinitionType) {
             yield 'struct';
-            foreach ($type->getProperties() as $name => $value) {
+            foreach ($type->getProperties() ?? [] as $name => $value) {
                 yield $name;
                 yield from $this->getValuesByType($value);
             }