inconsistent datatypes: expected - got CHAR - sql

Question

I have a senario where, I have the following Where Clause in my query. I am using Oracle 11g (11.2.0.1)
select * from aTable WHERE (attribute_name IN
(SELECT * FROM TABLE(CAST(:pbindvar AS CHARTABLETYPE)))or :pbindvar IS NULL)
The value for this bind variable is a array, but when I am trying to pass a null value to this bindvariable I am getting the following error
Expected - Got Char Error
Could anyone please let me know how am I supposed to pass a null value to this Bind variable so that the query will return me all the set of rows without applying the where condition.
Stack trace is as follows
ORA-00932: inconsistent datatypes: expected - got CHAR
00932. 00000 - "inconsistent datatypes: expected %s got %s"
*Cause:
*Action:
I tried the following as well but with no luck still getting the same exception
I tried with the conditional where clause still getting the same error
WHERE (CASE WHEN :pbindvar is NULL THEN 1
WHEN attribute_name IN (SELECT * FROM TABLE(CAST(:pbindvar AS CHARTABLETYPE))) THEN 1
ELSE 0
END) = 1
If I change the second condition where clause as mentioned below, the query is returning with all rows
WHERE (CASE WHEN :pbindvar is NULL THEN 1
WHEN attribute_name IN (:pbindvar) THEN 1
ELSE 0
END) = 1
Sample Query using hr schema
select * from Departments where department_name in (Select * from table(cast(:pbindvar as CHARTABLETYPE))) or :pbindvar is Null;
create query for type
create or replace type chartabletype
as table of VARCHAR2(4000);
The issue occurs only when the null is passed to bind variable