What is wrong with IF object_id sql in DNN format?

IF object_id(N'{databaseOwner}{objectQualifier}IX_swiztype'), 'C') IS NOT NULL)

BEGIN

ALTER TABLE {databaseOwner}{objectQualifier}swiztype ADD CONSTRAINT

       IX_swiztype PRIMARY KEY CLUSTERED

       (

       TypeID

       ) ON [PRIMARY]

END

GO

Above sql when add extension on the second pass (has IX_swiztype) it throws the below error

SQL Execution resulted in following Exceptions: System.Data.SqlClient.SqlException (0x80131904): There is already an object named 'IX_swiztype' in the database. Could not create constraint or index.

IF (OBJECT_ID('IX_swiztype', 'C') IS NOT NULL)

BEGIN

ALTER TABLE dbo.swiztype ADD CONSTRAINT

       IX_swiztype UNIQUE NONCLUSTERED

       (

       TypeName

       ) ON [PRIMARY]

END

GO

Above sql run direct is OK 

Command(s) completed successfully.