Why doesn't assume(x, 'real') exclude complex values when using plot(x^(1/3)) over a negative domain?

edit

Running the following commands:

assume(x,'real') plot(x^(1/3), -5, 5)

produces the following error: verbose 0 (3630: plot.py, generate_plot_points) WARNING: When plotting, failed to evaluate function at 100 points. verbose 0 (3630: plot.py, generate_plot_points) Last error message: 'negative number cannot be raised to a fractional power'

Shouldn't the assume command take care of this?

Thanks.