problem with custom functoid

I have create a biztalk 2004 “referenced” custom functoid that concatenates 2 strings, i’ve followed the microsoft example, i’ve compiled it, written it to the mapped extensions folder, installed it to gac and when i use it in a simple map and do a test map it throws the following error

The scripter functoid has no script type set, either external or inline, so proper code cannot be generated for it.

Has anybody any idea what this error means? Do I need to specify in my code a script type? The test map completes with this error and if i look at the xml generated it has everything else mapped ok except the fields that my functoids should have populated.

I specified category = functoidcategory.string … in case its relevant.

Liam