diff --git a/picoscope/ps2000a.py b/picoscope/ps2000a.py
index b87bc69..4858aaa 100644
--- a/picoscope/ps2000a.py
+++ b/picoscope/ps2000a.py
@@ -146,7 +146,8 @@ def __init__(self, serialNumber=None, connect=True):
     def _lowLevelOpenUnit(self, sn):
         c_handle = c_int16()
         if sn is not None:
-            serialNullTermStr = byref(create_string_buffer(sn))
+            serialNullTermStr = create_string_buffer(bytes(sn,
+                                                           encoding='utf-8'))
         else:
             serialNullTermStr = None
         # Passing None is the same as passing NULL