Why does the buck-boost converter output negative voltage?

Question

Electronics

I bought this buck-boost converter.

I decided to build it into an uninterruptible power supply to have a 5VDC output directly. Soldered to the battery (12 VDC), twisted the resistor, got 5 volts at the output. Everything seems to be working. Connected Raspberry Pi, also works. Touched the metal on the board - a small unpleasant shock. Useful to watch. It turned out that the converter produces -12 volts to ground, and -7 volts to plus. Difference: 5 volts, as ordered.
Is that how it's supposed to work? Or did I do something wrong? I would like zero ground.