Command block spawn issue

I'm in a super-flat world. When I typed in this command:

 /summon Chicken 1804 5 751 {CustomName:"Luke",CustomNameVisible:1,Attributes:[{Name:generic.movementSpeed,Base:0}],Invulnerable:1}

the command block output said "cannot summon object out of world". When I simply deleted the coordinates part of it the chicken spawned and everything was fine. I double the checked the coordinates and made sure they were correct and they were. What could be causing this and how can I fix it?