diff --git a/docker/utils/socket.py b/docker/utils/socket.py
index 4b328536..7ba95055 100644
--- a/docker/utils/socket.py
+++ b/docker/utils/socket.py
@@ -138,15 +138,21 @@ def consume_socket_output(frames, demux=False):
 
     # If the streams are demultiplexed, the generator yields tuples
     # (stdout, stderr)
-    out = [six.binary_type(), six.binary_type()]
+    out = [None, None]
     for frame in frames:
         # It is guaranteed that for each frame, one and only one stream
         # is not None.
         assert frame != (None, None)
         if frame[0] is not None:
-            out[0] += frame[0]
+            if out[0] is None:
+                out[0] = frame[0]
+            else:
+                out[0] += frame[0]
         else:
-            out[1] += frame[1]
+            if out[1] is None:
+                out[1] = frame[1]
+            else:
+                out[1] += frame[1]
     return tuple(out)
 
 
diff --git a/tests/integration/api_exec_test.py b/tests/integration/api_exec_test.py
index fd2f0eae..857a18cb 100644
--- a/tests/integration/api_exec_test.py
+++ b/tests/integration/api_exec_test.py
@@ -134,7 +134,7 @@ class ExecTest(BaseAPIIntegrationTest):
         # tty=True, stream=False, demux=True
         res = self.client.exec_create(id, cmd, tty=True)
         exec_log = self.client.exec_start(res, demux=True)
-        assert exec_log == (b'hello out\r\nhello err\r\n', b'')
+        assert exec_log == (b'hello out\r\nhello err\r\n', None)
 
         # tty=True, stream=True, demux=True
         res = self.client.exec_create(id, cmd, tty=True)
diff --git a/tests/unit/api_test.py b/tests/unit/api_test.py
index 0f5ad7c4..fac314d3 100644
--- a/tests/unit/api_test.py
+++ b/tests/unit/api_test.py
@@ -574,7 +574,7 @@ class TCPSocketStreamTest(unittest.TestCase):
 
     def test_read_from_socket_6(self):
         res = self.request(stream=False, tty=True, demux=True)
-        assert res == (self.stdout_data + self.stderr_data, b'')
+        assert res == (self.stdout_data + self.stderr_data, None)
 
     def test_read_from_socket_7(self):
         res = self.request(stream=False, tty=False, demux=False)