I got a j walking ticket?

I got a ticket for 100 dollars for j walking... The thing is I waited at the crosswalk till it said Walk and like 5 seconds later midway of crossing it changes back to dont cross and some cop rolls around the corner and calls me foward and says i was j walking and i tried to tell him what happened but he didnt listen to me... I mean like what the ****? Can the cops seriously do this? i mean its not my fault the damn cross light only lasted 3-5 seconds..