diff --git a/escpos/escpos.py b/escpos/escpos.py
index b55ee12..531ebbb 100644
--- a/escpos/escpos.py
+++ b/escpos/escpos.py
@@ -52,6 +52,7 @@ class Escpos(object):
         This function has to be individually implemented by the implementations.
 
         :param msg: message string to be sent to the printer
+        :type msg: bytes
         """
         pass
 
diff --git a/escpos/printer.py b/escpos/printer.py
index 85eb553..de54cf9 100644
--- a/escpos/printer.py
+++ b/escpos/printer.py
@@ -79,6 +79,7 @@ class Usb(Escpos):
         """ Print any command sent in raw format
 
         :param msg: arbitrary code to be printed
+        :type msg: bytes
         """
         self.device.write(self.out_ep, msg, self.interface)
 
@@ -143,6 +144,7 @@ class Serial(Escpos):
         """ Print any command sent in raw format
 
         :param msg: arbitrary code to be printed
+        :type msg: bytes
         """
         self.device.write(msg)
 
@@ -201,6 +203,7 @@ class Network(Escpos):
         """ Print any command sent in raw format
 
         :param msg: arbitrary code to be printed
+        :type msg: bytes
         """
         self.device.sendall(msg)
 
@@ -247,6 +250,7 @@ class File(Escpos):
         """ Print any command sent in raw format
 
         :param msg: arbitrary code to be printed
+        :type msg: bytes
         """
         if type(msg) is str:
             self.device.write(msg.encode())