How is USD Cash Yield ultra-low risk?

USD Cash Yield portfolio invests in short-dated US Treasury Bills, which are ultra-low-risk investments. US Treasury Bills are fully backed by the US government, which guarantees that your interest and principal payments will be paid on time. Their short maturities of 0 – 3 months make them less sensitive to interest rate movements and highly liquid, meaning that they can easily be converted to cash. The only risk associated with the USD Cash Yield portfolio would come from the highly unlikely scenario that the US government would default.