Workflow assembly. Trying to make a Lookup parameter optional

When creating a workflow assembly for CRM 3.0, CRM has no proper understanding of optional parameters. The best way to cope with this is to specify a default value in the workflow.config file, but I found a weird issue with parameters of type lookup.

What I expected to able to do was specify a default of {00000000-0000-0000-0000-000000000000} which would map to Guid.Empty. Creating a rule in Workflow Manager was fine, but when it came the running the rule, it failed prior to calling the assembly with the following error in Workflow Monitor:

'Error code = 800040005. The Microsoft CRM server is unavailable. There is a problem communicating with the Microsoft CRM server.' This is obviously an incorrect error message, given the message itself was written to the CRM server. Further investigation showed the problem was with the above Guid, and anything other than an empty Guid works fine, so I now use {FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF}