Passing a filename as a matlab function input

I am developing a verification environment where I want to be able to capture with file I/O some text status messages for each different test that is run.

I have a Matlab function block that handles the file I/O, one of the inputs to this function is the output filename for each test that is run. This value is set as a workspace variable, which gets updated each time a new test is run.

In the 'edit data' field for the function, I have set the scope of the filename input to 'Parameter'.

when I compile the design, I get the following error: "Expression 'TestName' for initial value of data 'TestName' (#28) must evaluate to a numeric or logical."

Is it possible to pass a filename set as a workspace variable into a function this way?

Many thanks in advance