error when trying to set RawValue('default')

Hi, I get the error "Only scalar values can be assigned to UI components" when I try to set the RawValue('default'). In the db, the default value for this field is NULL. The type of the field is varchar(32). What am I doing wrong? Thanks for your help. Best regards Maximilian Stölzle

The value is trying be assigned using Tag::setDefault or Tag::displayTo