Here's some code:
Function aba(nme As Guid)As String  
Return "hello world"
End Function
Dim qqq As Guid  
MsgBox 0 , aba(qqq)
Line 5 throws a runtime error: "Missing Close Parens", "Token found: ABA". That happens even if the function takes its only parameter by reference instead of by value.
If I replace every occurrence of "guid" in the code with, say, "quad", it works flawlessly.
I suppose that that is intended behavior, as per the thinBasic Help Manual on the topic of GUID (language->data types->GUID):
The GUID variable is typically used only as a parameter, rather than as a term in an expression.
However, a priori it was not obvious that the quote meant that GUID was not an accepted type for arguments. This has caused me some lossage. To prevent inflicting further lossage on any more unsuspecting users, I suggest changing the documentation to be more explicit about this issue.