diff --git a/packages/toolbox-core/src/toolbox_core/protocol.py b/packages/toolbox-core/src/toolbox_core/protocol.py
index 5f1cc86d..e2071ba2 100644
--- a/packages/toolbox-core/src/toolbox_core/protocol.py
+++ b/packages/toolbox-core/src/toolbox_core/protocol.py
@@ -41,7 +41,7 @@ def __get_type(self) -> Type:
         elif self.type == "array":
             if self.items is None:
                 raise Exception("Unexpected value: type is 'list' but items is None")
-            return list[self._items.to_type()]  # type: ignore
+            return list[self.items.__get_type()]  # type: ignore
 
         raise ValueError(f"Unsupported schema type: {self.type}")