Looking at the spread between 3 month commercial paper and the 3 month bill (using say, Fred), how/why (economically speaking) can this spread be negative? Are there some mechanics in the commercial paper markets at play that could make this happen? Or are there some idiosyncrasies with how these data-series/rates are sampled/constructed?
In general I would assume short term lending to private companies to be riskier than to the US government (except of course in a few extremely special cases, e.g. J&J and Microsoft).