Target Nearest Enemy

Hey all, looking for a little help with a macro. I need it to:

/targetnearestenemy
/cast Hunter's Mark(rank 1)
/petattack

When i put these all together in a macro, they don't work unless i keep hitting the button over and over. All the while I keep getting error messages like, "you don't have a target".

Please help this macro noob! thanks all.