Can't ALTER COLUMN TYPE in PostgreSQL "0A000: Cannot alter column "FIELDA" of relation "TABLEA"

Trying to run the below script:

ALTER TABLE SCHEMA.TABLEA
ALTER COLUMN FIELDA TYPE VARCHAR(5)

And I am getting the following error:

0A000: Cannot alter column "FIELDA" of relation "TABLE"

The field is currently an integer and contains no data, only nulls.