Negative One

My 5th grader came to me with a shockingly bad grade on a test about commas. I was sympathetic but still trying to process the information when he started talking about a math test.

"I got a negative two." 

"Wait. What?" 

"One of the questions was a," stopped listening momentarily here, "so really it was a negative one." 

I was quiet for a minute while I tried to figure that out. "Are you saying you got less than none right? One or two less than none? Is that even possible?" 

"Mom, it means I got one wrong," he said, exasperated. "Minus one. Maybe you shouldn't be worrying about my math tests." 

Maybe not, but I'm concerned for his grammar and sentence structure though.