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Contour Plot not working with pseudo-piecewise function

Hi, I'm fairly new to Sage and don't know if this is a bug or just my own limited understanding. I want to plot a function 'f(g1, g2)' but only in some relevant area of the plot. All other points should display/evaluate to zero. This should work fine in Octave or numpy but Sage for some reason gives me an error-message which I fail to understand.

Function assignment like so:

def f(g1, g2):
if g1*g2 > 0.0:
    if abs(g2) < 1/abs(g1):
        return sqrt(0.5*((1+g1*g2)*abs(g1+g2))/(sqrt((1.0-g1*g2)^3)*g1*g2))
    return 0

Then the Plot:

var('g1 g2')
contour_plot(f(g1,g2), (g1,-4,4), (g2,-4,4),plot_points=500, cmap='afmhot_r', colorbar=True, contours=srange(1,6,0.5))

and at last the error-message:

ValueError: zero-size array to ufunc.reduce without identity

Strangely, if I assign a single number to 'contours' (or leave it out to get the default value) the error-message does not show but instead I get an empty/white plot.

If I do not use a conditional/if-statement in my function-assignment the contour-plot shows just fine (except that it didn't do what I wanted). Does anyone know how to fix this or why it doesn't work or at least why I'm being silly for even trying it that way?

Thanks in advance