diff --git a/src/escpos/escpos.py b/src/escpos/escpos.py
index 237d960..04a0826 100644
--- a/src/escpos/escpos.py
+++ b/src/escpos/escpos.py
@@ -116,7 +116,7 @@ class Escpos(object):
 
     device = None
 
-    def __init__(self, profile=None, magic_encode_args=None, **kwargs):
+    def __init__(self, profile=None, magic_encode_args=None, **kwargs) -> None:
         """Initialize ESCPOS Printer.
 
         :param profile: Printer profile
diff --git a/tox.ini b/tox.ini
index ad1b518..ff30b29 100644
--- a/tox.ini
+++ b/tox.ini
@@ -53,5 +53,6 @@ deps = mypy
        types-PyYAML
        types-appdirs
        types-Pillow
+       types-pyserial
        jaconv
 commands = mypy src