WD20, 'Element unknown' error

I've started getting the following error

'WIN_Subcontractor_Selection' element unknown.

in the following code

sWindow is string = "WIN_Subcontractor_Selection"
stResource is stSubcontractor_Resource = Open({sWindow,indWindow},passed_SubcontractorID,passed_Type,passed_Job_Details)

which is in a class.

The element WIN_Subcontractor_Selection is present as a Window in the project, but for some reason is not visible to the class.

However this code does work:

stResource is stSubcontractor_Resource = Open(WIN_Subcontractor_Selection,passed_SubcontractorID,passed_Type,passed_Job_Details)

Anyone have an idea how to get the indirection version to work?

Rgds
Paul Turner