Damaged Wire Insulation
What everyone has said is fine, but here is an optional method I would use. First straighten the wire out as best you can. I usually us a pair of lineman's pliers our a heavy duty needle nose. Then you can purchase some shrink insulation from your local hardware store or electrical supplier and shrink a new piece of insulation around the nicked area. I generally shy away from using tape since you will find it hard to wrap and get a proper application in a small enclosed area. Also, depend on conditions tape has a tendency to come unraveled from time to time.

Good luck and stay safe.