Photoshop Script: exportDocument

I want to save my image as a transparent PNG and wrote a script which suddenly stopped working. I get this error message:

docExportOptions = new ExportOptionsSaveForWeb 

docExportOptions.format = SaveDocumentType.PNG //-24 //JPEG, COMPUSERVEGIF, PNG-8, BMP 
docExportOptions.transparency = true 
docExportOptions.blur = 0.0 
docExportOptions.includeProfile = false 
docExportOptions.interlaced = false 
docExportOptions.optimized = true 
docExportOptions.quality = 100 
docExportOptions.PNG8 = false 

docRef.exportDocument (theFile,ExportType.SAVEFORWEB,docExportOptions) 

Any ideas how to fix that? Or any other solutions to save the docRef as a transparent PNG?