What's wrong with a classic GDP joke?

Two doctoral students in economics were walking on the road and saw a lump of shit on the road. A said to B, if you can eat, I'll give you 50 million. B thought, what's so hard about eating shit? So you can get 50 million. It's worth it So I ate the shit. A had to go to the bank and lend 50 million yuan to B. Later, the more I thought about it, the more I felt it was uneconomical. Such a piece of shit will make him owe the bank 50 million for the rest of his life and then be a slave every day. It's a big loss. Fortunately, at this time, they saw another piece of shit. A said, if I can eat for fairness, shouldn't you give me 50 million, too? At this time, because of the nausea caused by eating dog shit just now, B wanted to get back at Xia Jia so that A could feel vomiting, and immediately agreed. A ate the second shit in one breath, so B took 50 million yuan from the bank to A .. They walked on, thinking as they walked, and the more they thought, the more depressed they became. Nothing happened, so they ate a piece of shit for no reason. They really couldn't figure it out and went to their professor. After listening to the reason, the professor said excitedly, one hundred million, one hundred million, if you eat two pieces of shit, you will contribute one hundred million GDP to the national economy.

Is there a mistake in the calculation of GDP?