Iphone5 connected to USB gives shock when touched to ear to speak

I usually charge my iphone5 with usb connected to laptop. past few days, i got a call while charging the phone with usb. I raised the phone to speak, as soon as it touched left ear, I got a decent electric shock. The shock was pretty powerful.

I ignored it first time, but after that, the same happened about 5-6 times. Now I do not speak while connected to usb. But I think this is not a normal behaviour of an iphone since other phones do not shock the user.

Can someone help me?