The Divorce Of The Anusa

The pasuk says that if a betrothed woman was raped, she is not punished as she is guiltless. Targum Yonason adds that her husband must divorce her. 

"לית לעלמה דין דקטול אלהן בעלה בגט פטורין יפטרנה" 

Why? Where does that come from????