SQL Server 2008 :: Creating Primary Key On Temporary Table?

Environment: Microsoft SQL Server Standard Edition (64-bit), 10.0.5520.0

I was doing a code review for another developer and came across this code:

CREATE TABLE dbo.#ABC
(
ReportRunTime DATETIME
,SourceID VARCHAR(3)
,VisitID VARCHAR(30)
,BaseID VARCHAR(25)

[Code] ....

This EXECUTES with no error or warning message.However, if I change this to CREATE the PK in an ALTER TABLE statement, I get the (expected by me) error:

CREATE TABLE dbo.#ABC
(
ReportRunTime DATETIME
,SourceID VARCHAR(3)
,VisitID VARCHAR(30)
,BaseID VARCHAR(25)
,OccurrenceSeqID INT

[code]...

==> Msg 8111, Level 16, State 1, Line 17 Cannot define PRIMARY KEY constraint on nullable column in table '#ABC'.

==> Msg 1750, Level 16, State 0, Line 17 Could not create constraint. See previous errors.

(note: As the #ABC table is an actual copy of a few of the columns in a "permanent" table, I will likely change the definition as follows such that the columns are defined to match the names / datatypes / NULLability:

SELECT TOP 0
CAST('01-01-1980' AS DATETIME) AS [ReportRunTime]
,SourceID
,VisitID
,BaseID
,OccurrenceSeqID

[Code] .....