Let be a finite Galois-algebra extension of a number field F, with group G. Suppose that is weakly ramified and that the square root of the inverse different is defined. (This latter condition holds if, for example, is odd.) Erez has conjectured that the class of in the locally free class group of is equal to the Cassou–Noguès–Fröhlich root number class associated with . This conjecture has been verified in many cases. We establish a precise formula for in terms of in all cases where is defined and is tame, and are thereby able to deduce that, in general, is not equal to .