Let $L/K$ be an odd degree Galois extension of number fields and set $G := \mathrm{Gal}(L/K)$. Let $A_{L/K}$ denote the square root of the inverse different. By a result of Erez $A_{L/K}$ is projective as a $ZG$-module if and only if $L/K$ is at most weakly ramified, i.e., for each ramified prime the second ramification subgroup (in lower numbering) is trivial. For such a weakly ramified odd degree Galois extension we define and study a canonical invariant in the relative algebraic $K$-group $K_0(ZG, QG)$ which projects to the class of $A_{L/K}$ in $K_0(ZG)$. Our results shed new light on a conjecture of Vinatier which predicts that $A_{L/K}$ is always a free $ZG$-module. This is joint work with David Burns and Carl Hahn.