Kotlin exposed - Reference is null - kotlin-exposed

Question

I have two tables: Users and UserPermissions
In the table Users I have an unique id.
In the table UserPermissions I created a reference to the Table Users.
Now when I want to get the UserPermissions from a User, I get a NullPointerException.
I searched around a little bit and found out, that the field permissions of the User is null.
How can this happen?
My code:
User:
object Users : IntIdTable("users") {
val firstName = varchar("first_name", 64)
val lastName = varchar("last_name", 64)
#EntityName("User")
class UserEntity(id: EntityID<Int>) : IntEntity(id) {
companion object : IntEntityClass<UserEntity>(Users)
var firstName by Users.firstName
var lastName by Users.lastName
val permissions by UserPermissionEntity referrersOn UserPermissions.user
}
UserPermission:
object UserPermissions : IntIdTable("user_permissions") {
val user: Column<EntityID<Int>> = reference("user_id", Users)
val permission = integer("permission")
}
class UserPermissionEntity(id: EntityID<Int>) : IntEntity(id) {
companion object : IntEntityClass<UserPermissionEntity>(UserPermissions)
var user by UserEntity referencedOn UserPermissions.user
var permission by UserPermissions.permission
}