diff --git a/routeros_api/sentence.py b/routeros_api/sentence.py
index a146910..0664886 100644
--- a/routeros_api/sentence.py
+++ b/routeros_api/sentence.py
@@ -2,6 +2,7 @@
 
 from routeros_api import exceptions
 from routeros_api import query
+from six import string_types
 
 
 response_re = re.compile(b'^!(re|trap|fatal|done)$')
@@ -56,6 +57,10 @@ def __init__(self, path, command, tag=None):
     def get_api_format(self):
         formated = [self.path + self.command]
         for key, value in self.attributes.items():
+            if isinstance(key, string_types):
+                key = key.encode()
+            if isinstance(value, string_types):
+                value = value.encode()
             formated.append(b'=' + key + b'=' + value)
         for query in self.queries:
             formated.extend(query.get_api_format())