What happens to the diffraction pattern when you have a small wavelength and large slit width in single slit interference?

Question

Say you have a light of 700nm shining through a diffraction grating with the separation of slits being 450nm. What pattern would you see?

The answer is given is a single, sharp peak.
I'm confused by this answer.

I know that d * sin(theta) = m * lambda

since theta = arcsin (m*lambda/d)

theta = arcsin(1.5556), which is invalid. So what is the reason for the sharp peak?