How come

$\displaystyle \left( \frac {-125}8 \right )^{ \frac 23 }$

is equal to

$\displaystyle \frac {25}4 $

I know that it can be shown using several exponent laws but I thought those were only valid for a rational exponent as long as the base is positive. I have been told that negative bases rased to rational exponents is undefined. Somebody please help me sort this conundrum out!