diff --git a/docker/client.py b/docker/client.py
index 8bd41ac3..bebabcdb 100644
--- a/docker/client.py
+++ b/docker/client.py
@@ -109,7 +109,7 @@ class Client(requests.Session):
             command = shlex.split(str(command))
         if isinstance(environment, dict):
             environment = [
-                '{0}={1}'.format(k, v) for k, v in environment.items()
+                u'{0}={1}'.format(k, v) for k, v in environment.items()
             ]
 
         if isinstance(mem_limit, six.string_types):