Modify table datatype from VARCHAR2(4000) to CLOB

[dpitts1979]

Gurus,

I'm trying to modify a datatype from varchar2(4000) to Clob in one of my tables and I'm getting the following error:

alter table gls_attr modify (attr_value CLOB);

ORA-22858: invalid alteration of datatype

I'm on 10G, shouldn't this work?