mop vs map

I taught English pronunciation today. It included the difference between "map" and "mop." This reminds me of my experience.

When I lived in Missouri, one of my friends visited my house. He went to the gas station near my house. He wanted to have a map of Missouri. And he said, "Can I have a map of Missouri?"

But, his "map" sounded like "mop." Therefore, the girl working there looked confused. Nobody can imagine what "map of Missouri" is...