Undefined value when there is nested model.

Question

I have two model with properties like :

export class Users {
UserId: string;
FirstName: string;
LastName: string;
}

export class Address{
AddressId: string;
Location: string;
PinCode: string;
}

Create one more model to call them, like 

export class AddCustomers {
UserEntity: Users;
AddressEntity: Address;
}

But whenever, i trying to access like this:

myObject:AddCustomers=new AddCustomers();
myObject.UserEntity.FirstName="Tim"; // it display error, Firstname not defined