$\displaystyle \int \frac {1}{x^4} dx$ from -1 to 2

My teacher said this is undefined

I did a calculation and it actually gave me somthing.

$\displaystyle \int \frac {1}{x^4} dx$

$\displaystyle = \int x^{-4}dx$

$\displaystyle =\frac 1{3} x^{-3} $

$\displaystyle f(x) = F(b) - F(a)$

$\displaystyle = [\frac 1{3} (2)^{-3}] - [\frac 1{3} (-1)^{-3}]$

$\displaystyle = - \frac 8{3} + \frac 1{3}$

$\displaystyle = -\frac 7{3}$

My question is why the answer is undefined and not $\displaystyle -\frac 7{3}$?