Index: utils/lit/lit/util.py
===================================================================
--- utils/lit/lit/util.py
+++ utils/lit/lit/util.py
@@ -10,6 +10,8 @@
 
 def to_bytes(str):
     # Encode to UTF-8 to get binary data.
+    if isinstance(str, bytes):
+        return str
     return str.encode('utf-8')
 
 def to_string(bytes):
@@ -200,6 +202,8 @@
         If the timeout is hit an ``ExecuteCommandTimeoutException``
         is raised.
     """
+    if input is not None:
+        input = to_bytes(input)
     p = subprocess.Popen(command, cwd=cwd,
                          stdin=subprocess.PIPE,
                          stdout=subprocess.PIPE,