binary key in dictionary

[cerr]

Hi,

In my application I have followingf lines:
                print curr_mac
                print hexlify(buf)
                binmac = unhexlify(curr_mac)
                tmpgndict[binmac] += buf
curr_mac being a 3Byte MAVC address in ASCII and I want to populate a dictionary where the value(buf) is indexed by binary mac.

I get this in my code:

Traceback (most recent call last):
  File "gateway.py", line 2485, in <module>
    main()
  File "gateway.py", line 2459, in main
    cloud_check()
  File "gateway.py", line 770, in cloud_check
    gnstr_dict[src] = gn_from_cloud(curr_mac)
  File "gateway.py", line 2103, in gn_from_cloud
    tmpgndict[binmac] += "HELLO"
KeyError: '\x04\xeeu'

but then again, the following works fine in the python interpreter:
>>> mac = '04ee75'
>>> dat = '2a0001016d03c400040001000a'
>>> mydict = {}
>>> mydict[unhexlify(mac)]=dat
>>> print mydict
{'\x04\xeeu': '2a0001016d03c400040001000a'}

I really seem to do something wrong and can't see what it is. Can anyone help me further here?

Thank you very much!
Ron