SQL & PL/SQL :: Materialized View - Zero Length Not Allowed?

I am trying to create one Materialized view from a Select Statement and I am getting the following error

SQL Error: ORA-01723: zero-length columns are not allowed

I have checked the table and found nothing like varchar2(0) or char(0) , but the column in my select statment may get a Null value. Will it result in error like above ?

Normal View : I have created this because I think a materialized view would not be created if we have a Sub query.

CREATE OR REPLACE VIEW mx_test
AS
SELECT t."EXTN_SERVICE_REQUEST_NO",
t."EXTN_SERVICE_TYPE",

[Code]....

Materialized View :

CREATE materialized VIEW mx_ashok
AS
SELECT * FROM mx_test

Error :

Error starting at line 1 in command:
CREATE materialized VIEW mx_ashok
AS
SELECT * FROM mx_test
Error at Command Line:3 Column:16
Error report:
SQL Error: ORA-01723: zero-length columns are not allowed
01723. 00000 - "zero-length columns are not allowed"
*Cause:
*Action:

Do I need to use NVLs for all columns which may result in NULL value ?