Change the CharField to TextField

Question

I want convert one of my field type from CharField to TextField

I changed as followings:

From:
project = models.CharField('Project',max_length=500)        

To:
project = models.TextField('Project',blank=True)

And after I run the migrate cmd , I met the following errors:

django.db.utils.OperationalError: (1170, "BLOB/TEXT column 'project' used in key specification without a key length")

I performed the steps mentioned in Django: Convert CharField to TextField with data intact

MySQL:
BEGIN;
    ALTER TABLE TestLog
        MODIFY failed_reqs TEXT NULL,
        MODIFY passed_reqs TEXT NULL;
COMMIT;

Bug met the same errors:

mysql> ALTER TABLE ct_task
    -> MODIFY project TEXT NULL;
ERROR 1170 (42000): BLOB/TEXT column 'project' used in key specification without a key length

Anyone can help how can i do this conversion ? many thanks