The problem seems to me that there is NullLiteralExpression and LiteralExpression. If you manually create an LiteralExpression everything works out just fine. Also i doubt that any Type or AttributeConverter can currently replace null-values as this is what NullLiteralExpression uses to render the SQL:

	public String render(RenderingContext renderingContext) {
		return "null";
	}

and CriteriaUpdateImpl#set(javax.persistence.criteria.Path<Y>, X) represents null by NullLiteralExpression. On a side-node this also leads to generation of more distinct queries being turned into prepared statements.