Linear regression in ML

Question

If my model is perfect then that means $SSE = 0$,

which means $SST = SSR$ as $SST = SSR + SSE$,

which means $R^2 = 1 - 1 = 0$, as $R^2 = 1 - SSR/SST$,

but $R^2$ should have given me $1$ and not $0$,

I don't know what I am not understanding.