GORM Promoter: define a valid foreign key for relations or implement the Valuer/Scanner interface

I have been trying to use GORM foreign keys and I have tried everything but I keep getting the error Promoter: define a valid foreign key for relations or implement the Valuer/Scanner interface.

Here is my code:

type Site struct {
    gorm.Model
    ID       int64 `gorm:"primaryKey"`
    Domain   string
    Theme    string
    Promoter Promoter `gorm:"foreignKey:PromoterID;references:ID"`
}

type Promoter struct {
    gorm.Model
    ID     int64 `gorm:"primaryKey"`
    Name   string
    Owners []User `gorm:"many2many:user_promoters;"`
}