diff --git a/picoscope/ps6000.py b/picoscope/ps6000.py
index 5923558..77a6fd1 100644
--- a/picoscope/ps6000.py
+++ b/picoscope/ps6000.py
@@ -59,7 +59,7 @@
 # float is always defined as 32 bits
 # double is defined as 64 bits
 from ctypes import byref, POINTER, create_string_buffer, c_float, \
-    c_int16, c_int32, c_uint32, c_uint64, c_void_p
+    c_int8, c_int16, c_int32, c_uint32, c_uint64, c_void_p
 from ctypes import c_int32 as c_enum
 
 from picoscope.picobase import _PicoscopeBase
@@ -198,7 +198,10 @@ def _lowLevelCloseUnit(self):
 
     def _lowLevelEnumerateUnits(self):
         count = c_int16(0)
-        m = self.lib.ps6000EnumerateUnits(byref(count), None, None)
+        serials = c_int8(0)
+        serialLth = c_int16(0)
+        m = self.lib.ps6000EnumerateUnits(byref(count), byref(serials),
+                                          byref(serialLth))
         self.checkResult(m)
         # a serial number is rouhgly 8 characters
         # an extra character for the comma