Trouble with Curve Fitting a Custom Fit Function

Function SingleSlit(w,theta) : FitFunc
    Wave w
    Variable theta

    //CurveFitDialog/ These comments were created by the Curve Fitting dialog. Altering them will
    //CurveFitDialog/ make the function less convenient to work with in the Curve Fitting dialog.
    //CurveFitDialog/ Equation:
    //CurveFitDialog/ f(theta) = y0+ (I0*(((sin(((3.14*a)/lambda)*sin(theta)))/(((3.14*a)/lambda)*sin(theta)))^(2)))
    //CurveFitDialog/ End of Equation
    //CurveFitDialog/ Independent Variables 1
    //CurveFitDialog/ theta
    //CurveFitDialog/ Coefficients 4
    //CurveFitDialog/ w[0] = I0
    //CurveFitDialog/ w[1] = a
    //CurveFitDialog/ w[2] = lambda
    //CurveFitDialog/ w[3] = y0

    Variable numerator = sin(((3.14*w[1])/w[2])*sin(theta))
    Variable denominator = ((3.14*w[1])/w[2])*sin(theta)
    Variable value = w[3]+ (w[0]*(((numerator)/(denominator))^(2)))
    if (numtype(value) != 0)
        print "Uh oh"
    endif
    return value
End