upsampling and plotting a signal in matlab

I want to upsample by 5 a signal in frequency domain, and then plot(stem) it. I figured how to upsample,

>Fk=(1/5)*upsample(ak_new,5)

now this creates a vector that is 5 times bigger than the original one, and I need to take the inverse fourier series of this signal

>Fn=(Fk*(exp((1i*2*pi/N*n'*n))));

where n is a sample vector (-1000:1000), as you can see I can't make the transformation since n is not the same size as Fn anymore. how can I solve this? thanks