Question On Evaluating Expression

Hi all€”I'm new to expressions, and am trying to write one where I can evaluate if a field is null, to handle it by inserting a default value if a null value is found. Given this table definition:

CREATE TABLE [dbo].[Meter Readings_stage](
[Meter System ID] [varchar](8) NOT NULL,
[Reading Date/Time] [datetime] NOT NULL,
[Meter Status] [varchar](50) NULL,
[Totalizer] [float] NULL,
[Secondary Totalizer] [real] NULL CONSTRAINT [DF_Meter Readings_Secondary Totalizer2] DEFAULT (NULL),
[Demand] [real] NULL CONSTRAINT [DF_Meter Readings_Demand2] DEFAULT (NULL),
[Demand Date/Time] [datetime] NULL CONSTRAINT [DF_Meter Readings_Demand Date/Time2] DEFAULT (NULL),
[Reading Type] [varchar](30) NULL CONSTRAINT [DF_Meter Readings_Reading Type2] DEFAULT (NULL),
[Comments] [varchar](200) NULL CONSTRAINT [DF_Meter Readings_Comments2] DEFAULT ('None'),
[Checked?] [bit] NULL CONSTRAINT [DF_Meter Readings_Checked ?2] DEFAULT ((1)),
[Estimate Required] [bit] NULL CONSTRAINT [DF_Meter Readings_Estimate Required2] DEFAULT ((0)),
[Hold] [bit] NULL CONSTRAINT [DF_Meter Readings_Hold2] DEFAULT ((0)),
[Processed?] [bit] NOT NULL CONSTRAINT [DF_Meter Readings_Processed2] DEFAULT ((0))

I am importing data from a CSV file where any of the fields can be null. The format is as follows:

[Meter System ID],[Reading Date],[Reading Time],[Totalizer],[Demand],[Demand Date],[Demand Time], [Reading Type]

I need to handle it something like the following in a derived column transformation, where:

[Meter System ID]: string [DT_STR]
[Reading Date]: database date [DT_DBDATE]
[Reading Time]: database time [DT_DBTIME]
[Totalizer]: float [DT_R4]
[Demand]: float [DT_R4]
[Demand Date]: database date [DT_DBDATE]
[Demand Time]: database time [DT_DBTIME]
[Reading Type]: string [DT_STR]

This is what I have built so far:
NULL((DT_DBDATE)("Reading Date")) ? (DT_DBDATE)(GETDATE()) : (DT_DBDATE)("Reading Date")

I am getting an error on the cast conversion, however. Where is the error in the evaluation?

Thanks,
Jon