How a Large Customer Can Kill a Startup
I've worked for ten startups during my career.  Even before the last couple, I had learned to recognize the "one big customer will bring us to the promised land" pattern as one of the surest indicators of failure.  Those customers always end up sucking all the resources away from a more generally appealing product direction, and they never come through financially, so your development ends up months or years behind where it should have been with nothing to show for it. 
 Some big companies deliberately take advantage of startups to do free work, with never any intent to buy the product itself.  If you see this happening at your company, my advice is to start planning your next move.