http://quant.stackexchange.com/questions/8857/quantlib-error-with-qlpiecewiseyieldcurvedata-on-qlpiecewiseyieldcurve-with
It's an issue with the LogLinear interpolation; while trying to find the correct rate for the 1-week node, the bootstrapper wanders unchecked into a region of negative rates and the logarithms blow up.
Moved to https://github.com/lballabio/QuantLib/issues/363