MySQL alter table modify column failing at rows with null values

Question

I have a table with about 10K rows, which I am trying to alter so that the field fielddelimiter is never null. I am attempting to do an alter statement, expecting a

Answer 1

First remove any null values

UPDATE merchant_ftp_account SET fielddelimiter='t' WHERE fielddelimiter IS NULL;

Then

ALTER TABLE merchant_ftp_account MODIFY COLUMN `fielddelimiter` char(1) NOT NULL DEFAULT 't';