diff --git a/compose_viz/models/device.py b/compose_viz/models/device.py
index e777a3d..80d6b1e 100644
--- a/compose_viz/models/device.py
+++ b/compose_viz/models/device.py
@@ -1,6 +1,8 @@
+from typing import Optional
+
 
 class Device:
-    def __init__(self, host_path: str, container_path: str, cgroup_permissions: str = None):
+    def __init__(self, host_path: str, container_path: str, cgroup_permissions: Optional[str] = None):
         self._host_path = host_path
         self._container_path = container_path
         self._cgroup_permissions = cgroup_permissions
@@ -12,7 +14,7 @@ class Device:
     @property
     def container_path(self):
         return self._container_path
-    
+
     @property
     def cgroup_permissions(self):
-        return self._cgroup_permissions
\ No newline at end of file
+        return self._cgroup_permissions
diff --git a/compose_viz/models/service.py b/compose_viz/models/service.py
index eed285c..8e578c2 100644
--- a/compose_viz/models/service.py
+++ b/compose_viz/models/service.py
@@ -19,7 +19,7 @@ class Service:
         extends: Optional[Extends] = None,
         cgroup_parent: Optional[str] = None,
         container_name: Optional[str] = None,
-        devices: Optional[Device] = [],
+        devices: List[Device] = [],
         env_file: List[str] = [],
         expose: List[str] = [],
         profiles: List[str] = [],