diff --git a/smpplib/command.py b/smpplib/command.py
index b19b0c6..64fbf41 100644
--- a/smpplib/command.py
+++ b/smpplib/command.py
@@ -498,8 +498,8 @@ class DataSM(Command):
         'callback_num': Param(type=ostr, min=4, max=19),
         'callback_num_pres_ind': Param(type=int, size=1),
         'callback_num_atag': Param(type=str, max=65),
-        'source_subaddress': Param(type=str, min=2, max=23),
-        'dest_subaddress': Param(type=str, min=2, max=23),
+        'source_subaddress': Param(type=ostr, min=2, max=23),
+        'dest_subaddress': Param(type=ostr, min=2, max=23),
         'user_response_code': Param(type=int, size=1),
         'display_time': Param(type=int, size=1),
         'sms_signal': Param(type=int, size=2),
@@ -684,8 +684,8 @@ class SubmitSM(Command):
         'privacy_indicator': Param(type=int, size=1),
         'callback_num': Param(type=ostr, min=4, max=19),
         'callback_num_pres_ind': Param(type=int, size=1),
-        'source_subaddress': Param(type=str, min=2, max=23),
-        'dest_subaddress': Param(type=str, min=2, max=23),
+        'source_subaddress': Param(type=ostr, min=2, max=23),
+        'dest_subaddress': Param(type=ostr, min=2, max=23),
         'user_response_code': Param(type=int, size=1),
         'display_time': Param(type=int, size=1),
         'sms_signal': Param(type=int, size=2),
@@ -785,8 +785,8 @@ class DeliverSM(SubmitSM):
         'payload_type': Param(type=int, size=1),
         'message_payload': Param(type=ostr, max=260),
         'callback_num': Param(type=ostr, min=4, max=19),
-        'source_subaddress': Param(type=str, min=2, max=23),
-        'dest_subaddress': Param(type=str, min=2, max=23),
+        'source_subaddress': Param(type=ostr, min=2, max=23),
+        'dest_subaddress': Param(type=ostr, min=2, max=23),
         'language_indicator': Param(type=int, size=1),
         'its_session_info': Param(type=int, size=2),
         'network_error_code': Param(type=ostr, size=3),