Oracle Database Discussions

I am very puzzled by the error I'm getting.

I've successfully created the below two tables (disregard my use of "char" variable type; beyond the scope of my question):

create table Test_req (
Req_ID varchar2(4) not null,
Req_Comment varchar2(50) null
);

create table Test_n_cycle (
Req_ID varchar2(4) not null,
Test_ID varchar2(3) not null,
Cycle_cd char(1) not null,
Cycle_Comment varchar2(150) null
);


Then I successfully performed the two alter statements:

Alter table test_n_cycle
add constraint
foreign key (req_id) references test_req (req_id)
Error report:
SQL Error: ORA-00902: invalid datatype
00902. 00000 - "invalid datatype"

Is this because req_id is also defined in the Primary key definitions on these
two tables, thus making the field not valid for referential constraint definition?