GetContainerAttribute - not recognizing field?

[Wickerman]

I am struggling to do something simple, which is to define a field that will display the filename of files stored ion a container field called 'Document'.  I am using the  GetContainerAttribute(sourceField;attributeName) function, but I can't seem to get the engine to recognize the field, which I'm entering by double-clicking it in the field list!  I've tried using different attribute names, and tried doing it on other container fields in different files, but there's something I'm just not getting here . . . can anyone spot my problem in this image of my attempt? 

[comment]

Your formula is looking for a field named "filename" to get a value for the attributeName parameter from there. If you want to supply the value directly as a text constant, you must enclose it in quotes:

GetContainerAttribute ( Document ; "filename" )

 

[Wickerman]

Ah -- that would be it.  Thanks.