Maxima asks about sign of integration(!) variable



Hi,

when I do:

expr : sqrt(1+(4*x^(1/3)-1/16*x^((-1)/3))^2);
integrate(expr, x, -8, 512);

Maxima asks "Is x positive or negative?".
Why does it ask that? x is the integration variable ... it shouldn't ask 
anything about it.

Also, when I answer "positive;", Maxima keeps asking the same question 
again two more times.
Only then, it gives me an answer.

The whole asking thing seems very buggy to me ...

David