Creating unique index with peewee for table with foreign key

Say that I have a table called TBL_ACCOUNT which contains all of my users, and a table called TBL_EMAIL_SUBSCRIPTION where it contains the feeds that a user is subscribed to. I'm trying to make it so that there can only be one entry of each user+feed combination, so user1 can only be subscribed to feed1 once, but user1 can be subscribed to feed1 and feed2 simultaneously.

This is what my model looks like:

class TBL_ACCOUNT(BaseModel):
    USERNAME = CharField(unique=True)
    PASSWORD = CharField()
    EMAIL = CharField(unique=True)

class TBL_EMAIL_SUBSCRIPTION(BaseModel):
    USER = ForeignKeyField(TBL_ACCOUNT)
    FEED = CharField()

    class Meta:
        indexes = (("USER_id", "FEED", True))

I have also tried just using "USER" for the indexes, but that didn't work out as the database still received duplicates.