That Contract Was Just For Funsies, Then?

After sending two invoices for payment, I sent another and called the client when I got the receipt saying that they had received it. 

Client: “Why are you calling me?”

Me: “You haven’t paid, and this is the third invoice I’ve sent.”

Client: “It’s even more than the last one!”

Me: “Yes. The contract you signed stated that I would add a fee for late payment.”

Client: “You mean I have to actually pay you? I thought you were joking!”

Me: “What on earth made you think that?”

Client: “You’re a freelancer!”

Me: “And…?”

Client: “Well, you work for free! If you were supposed to be paid, you’d be called a paidlancer or something!”