Add a CSS class to Css LiftScreen field

I just tried to add a css class to a mirrored Record field with the result that the CSS is not added to the resulting html.

object assoc extends ScreenVar(Assoc.createRecord)

val name: Field{type ValueType = String} = field(assoc.is.name, "class" -> "span1" , flagFieldError(name))

I tried with different attributes that do not occur in the screen CSS but that didn't help. It works fine for screen fields that do not mirror any Record fields.