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Interesting:

sage: float(-1)^(1/3)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)

/Users/grout/projects/HLA2/<ipython console> in <module>()

/Users/grout/sage-trees/sage-5.0.beta1/local/lib/python2.7/site-packages/sage/rings/rational.so in sage.rings.rational.Rational.__pow__ (sage/rings/rational.c:17026)()

ValueError: negative number cannot be raised to a fractional power
sage: RDF(-1)^(1/3)
NaN

In other words, having an RDF just doesn't give the error.