How to handle NULLs in external tables

Hi all,
I have the following tab-delimited data in my comps.txt file:

1	31	248161	"Company1"
	

2	31	248221	"Company2"
	

2	31	248301	"Company3"
	

		248323	"Company4"
	

		248403	"Company5"
	

1	"12"	248406	Company6
	

The following external table was created against the file:

CREATE TABLE EXT_COMPS
(
  C1  VARCHAR2(100 BYTE),
  C2  VARCHAR2(100 BYTE),
  C3  VARCHAR2(100 BYTE),
  C4  VARCHAR2(100 BYTE)
)
ORGANIZATION EXTERNAL
  (  TYPE ORACLE_LOADER
     DEFAULT DIRECTORY MYDIR
     ACCESS PARAMETERS 
       ( records delimited by newline
     badfile MYDIR:'COMPS%a_%p.bad'
     logfile MYDIR:'COMPS%a_%p.log'
     fields terminated by 0x'09'
     OPTIONALLY ENCLOSED BY '"'
     missing field values are null
 ( C1 CHAR,
   C2 CHAR ,
   C3 CHAR ,
   C4 CHAR) )
     LOCATION (MYDIR:'comps.txt')  )
REJECT LIMIT UNLIMITED