Import table data gives "ORA-01400: Cannot insert NULL into (schema.table.column)" (4225542)

When trying to import from and Excel sheet into a table using Toads Database menu | Import | Import table data, it gives the error:

ORA-01400: Cannot insert NULL into (schema.table.column)

Run the Import Table Data. The set up for the wizard works fine, and the data shows up correctly in the preview. But in the end when trying to execute the import, it errors out and the first row of data is not imported.

Sample Table:
CREATE TABLE SAMPLE_TYPE
(
COL1_ID VARCHAR2(1 BYTE) NOT NULL,
COL2_TYPE_DSC VARCHAR2(15 BYTE)
);

Sample Data:

A Actual
B Budget

But, in the excel file, if the data in the first row is changed from something like A to AA, the import works without any errors, and ends up inserting a single A instead of double. It does not prompt to trim the imported value.