Odd virtual column error

Hi,
Oracle version 11.1.0.6

Any ideas why when I run this ddl:

CREATE TABLE PILOT1_METADATA
(
 PATENT_SK NUMBER(9) NOT NULL
,COLLECTION_ID VARCHAR2(8) NOT NULL
,PDOC_COUNTRY CHAR(2) NOT NULL
,PDOC_SERIAL NUMBER(12) NOT NULL
,PDOC_KIND VARCHAR2(2) NOT NULL
,KI_SHORT CHAR(4) GENERATED ALWAYS AS (SUBSTR(PDOC_KIND,1,1)) VIRTUAL
,PUBLICATION_DATE DATE NOT NULL
,PUBLICATION_WEEK NUMBER(6) NOT NULL
,PUBLICATION_YEAR NUMBER(4) NOT NULL
,APPLICATION_NUMBER VARCHAR2(15) NOT NULL
,APPLICATION_DATE DATE NOT NULL
,STATUS VARCHAR2(3)
,MOD_DATE DATE

)
/

the result is:

ORA-12899: value too large for column "KI_SHORT" (actual: 1, maximum: 4)

I changed the column definition of PDOC_KIND to CHAR(4) and the table builds and then an insert creates the correct virtual data.

Thanks

Pete

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