As Germany issued 10-year bonds with negative yield for the first time in history on Wednesday, Jim Cramer had a word of caution for Wall Street.

"The German 10-year right now with its eyesore of a negative yield is therefore the WORST that money can buy. Whoever is pushing this lunatic piece of paper to people and whoever is buying them are making a huge mistake," the "Mad Money" host said.

Approximately 33 percent of all government bonds worldwide now have negative yields, according to The Wall Street Journal. In other words, investors that hold these bonds until maturity will lose money.

Cramer heard many investors use politics as an explanation, stating that negative rates reflect a failed tyranny of the European Union, or that Germany should be borrowing interest-free money to finance projects that could put people to work.

"My job is not to talk about European politics or debate monetary policy," Cramer said. "I'm on a mission to help you try to make money."

Cramer started selling securities at Goldman Sachs in 1984, when the 30-year Treasury yielded 13.5 percent. He loved stocks since he was a little boy, and couldn't wait to have his clients buy the stocks of Coca-Cola, Pfizer and Merck.

However, his wiser colleagues had a different idea. They explained risk and reward, and told him he could help people by selling Treasury bonds because they were virtually risk-free and that the 13.5 percent yield was juicy.

So, Cramer put away his risky desires and sold Treasurys with the understanding that what really mattered was how well a client did because if the client was happy, they would be a client for life.

"Those had to be the best pieces of risk-reward paper — no risk all reward — that I've ever seen. Sure, the stocks I liked back then went up gigantically, but remember, I was only selling to rich people and you only need to get rich once," Cramer said.

Now that the German 10-year yields less than nothing, Cramer imagined what he would say to someone who just got out of the training program in the German version of Goldman Sachs.

He would say that it is the worst piece of paper on earth to sell. It would doom clients to lose money, something that was a clear cardinal sin of what he was told to do 32 years ago.

"Forget the politics. A bad investment is a bad investment," Cramer said.

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