Stalling in first

I have a '76 kz400, the bike starts fine, was running and driving fine, but I just went to ride and when I put it in first and release the clutch it stalls every time. It feels like the bike is in first as it starts to take me, but then loses power and dies. 

Thanks for any help,
Blaze Watson

Not sure dont have a 400 but most kawas have a  safety switch on kickstand and clutch lever . They fail often . I would check those first