Taken directly from the immediate window:

reader["DateDue"] as DateTime? yields:

'reader["DateDue"] as DateTime?' threw an exception of type 'System.NullReferenceException'
Data: {System.Collections.ListDictionaryInternal}
HResult: -2147467261
HelpLink: null
InnerException: null
Message: "Object reference not set to an instance of an object."
Source: null
StackTrace: null
TargetSite: null

(DateTime?)reader["DateDue"] yields:

{1/26/2015 12:00:00 AM}
Date: {1/26/2015 12:00:00 AM}
Day: 26
DayOfWeek: Monday
DayOfYear: 26
Hour: 0
Kind: Unspecified
Millisecond: 0
Minute: 0
Month: 1
Second: 0
Ticks: 635578272000000000
TimeOfDay: {System.TimeSpan}
Year: 2015

and for reference, reader["DateDue"] yields:

{1/26/2015 12:00:00 AM}
Date: {1/26/2015 12:00:00 AM}
Day: 26
DayOfWeek: Monday
DayOfYear: 26
Hour: 0
Kind: Unspecified
Millisecond: 0
Minute: 0
Month: 1
Second: 0
Ticks: 635578272000000000
TimeOfDay: {00:00:00}
Year: 2015

Is this a bug? If directly casting to DateTime? works then casting with as DateTime? should work too.

I did find a work around for this by using reader.GetDateTime(reader.GetOrdinal("DateDue")) as DateTime?. Nevermind, that doesn't handle nulls well. Anyway there's myriad ways to paper around this oddity.

Repo demonstrating the issue can be found here: https://github.com/jjoedouglas/exceptionAsDatetime