diff --git a/paramiko/channel.py b/paramiko/channel.py
index d715bb8..6a8a798 100644
--- a/paramiko/channel.py
+++ b/paramiko/channel.py
@@ -969,7 +969,7 @@ class Channel (object):
                 self.transport._send_user_message(m)
 
     def _feed(self, m):
-        if isinstance(m, bytes_type):
+        if isinstance(m, bytes_types):
             # passed from _feed_extended
             s = m
         else:
diff --git a/paramiko/common.py b/paramiko/common.py
index 9161f30..5d7ab31 100644
--- a/paramiko/common.py
+++ b/paramiko/common.py
@@ -140,7 +140,7 @@ else:
 
 
 def asbytes(s):
-    if not isinstance(s, bytes_type):
+    if not isinstance(s, bytes_types):
         if isinstance(s, string_types):
             s = b(s)
         else:
diff --git a/paramiko/py3compat.py b/paramiko/py3compat.py
index e4830da..fd6488d 100644
--- a/paramiko/py3compat.py
+++ b/paramiko/py3compat.py
@@ -1,6 +1,6 @@
 import sys
 
-__all__ = ['PY3', 'string_types', 'integer_types', 'text_type', 'bytes_type', 'long', 'input', 'bytestring', 'byte_ord', 'byte_chr', 'byte_mask', 'b', 'u', 'b2s', 'StringIO', 'BytesIO', 'is_callable', 'MAXSIZE', 'next']
+__all__ = ['PY3', 'string_types', 'integer_types', 'text_type', 'bytes_types', 'long', 'input', 'bytestring', 'byte_ord', 'byte_chr', 'byte_mask', 'b', 'u', 'b2s', 'StringIO', 'BytesIO', 'is_callable', 'MAXSIZE', 'next']
 
 PY3 = sys.version_info[0] >= 3
 
@@ -8,9 +8,9 @@ if PY3:
     import collections
     import struct
     string_types = str
-    integer_types = int
     text_type = str
-    bytes_type = bytes
+    bytes_types = bytes
+    integer_types = int
     long = int
     input = input
 
@@ -65,9 +65,9 @@ if PY3:
     MAXSIZE = sys.maxsize       # NOQA
 else:
     string_types = basestring
-    integer_types = (int, long)
     text_type = unicode
-    bytes_type = str
+    bytes_types = str
+    integer_types = (int, long)
     long = long
     input = raw_input
 
diff --git a/paramiko/sftp.py b/paramiko/sftp.py
index 11aa3cb..4186460 100644
--- a/paramiko/sftp.py
+++ b/paramiko/sftp.py
@@ -142,7 +142,7 @@ class BaseSFTP (object):
         return
 
     def _read_all(self, n):
-        out = ''
+        out = bytes()
         while n > 0:
             if isinstance(self.sock, socket.socket):
                 # sometimes sftp is used directly over a socket instead of
@@ -176,7 +176,7 @@ class BaseSFTP (object):
         x = self._read_all(4)
         # most sftp servers won't accept packets larger than about 32k, so
         # anything with the high byte set (> 16MB) is just garbage.
-        if x[0] != '\x00':
+        if byte_ord(x[0]):
             raise SFTPError('Garbage packet received')
         size = struct.unpack('>I', x)[0]
         data = self._read_all(size)
diff --git a/paramiko/sftp_client.py b/paramiko/sftp_client.py
index 7703246..22a9a04 100644
--- a/paramiko/sftp_client.py
+++ b/paramiko/sftp_client.py
@@ -42,7 +42,7 @@ def _to_unicode(s):
     """
     try:
         return s.encode('ascii')
-    except UnicodeError:
+    except (UnicodeError, AttributeError):
         try:
             return s.decode('utf-8')
         except UnicodeError:
@@ -698,7 +698,7 @@ class SFTPClient (BaseSFTP):
                     msg.add_int(item)
                 elif isinstance(item, long):
                     msg.add_int64(item)
-                elif isinstance(item, string_types):
+                elif isinstance(item, (string_types, bytes_types)):
                     msg.add_string(item)
                 elif isinstance(item, SFTPAttributes):
                     item._pack(msg)