Nominal interest rates are constrained by an effective lower bound, but the level of the lower bound is uncertain. This paper uses a simple shadow rate term structure model to study how lower bound uncertainty affects long-term interest rates. The main result is that a decline in lower bound uncertainty, in the sense of a mean-preserving contraction of the lower bound distribution, is associated with a drop in expected future short rates. The effect on the variance of future short rates, and hence the term premium, is ambiguous. A calibration to Canadian data suggests that a decline in lower bound uncertainty is associated with a modest drop in long-term interest rates.