A weapons peddler and a wise man

Once upon time, there was a weapons peddler selling his goods in the streets. On that particular day, he was selling his best shields and swords. In an attempt to grab the attention of the public in the bustling streets, he promoted his weapons by shouting out loud the best features that his shields and swords possessed.

“My swords are the best! It is able to slice through anything in this world!” He went on promoting about the greatness of his swords.

“My shields are second to none! It is able to withstand any weapons in this world!” He continued to promote his shields.

A wise man happened to walk by him at that time. The wise man turned to the peddler and asked, “Young man, if I may, I would like to ask you just one question regarding your fine weapons.”

The peddler, visibly happy as he managed to secure the attention of somebody replied, “Hohoho, go ahead, sir. My weapons are the best! You won’t regret purchasing them!”

The wise man then went on to ask his question, “Young man, what would happen if you use your sword to slice your shield, since your sword is able to slice through anything while your shield is impervious to any attacks?”

The peddler’s face turned pale as he was at a loss for words. Furthermore, the question was asked in front of a very large crowd…

What is the moral of the story? Post your answers!