$m_u=0$ has long been proposed as a solution to the strong CP problem. While this solution is sometimes thought to have been excluded, it
is actually still ill-defined. In this work, we study the mass dependence of the physical observable $\chi_t$, the topological susceptibility.
Assigning an unphysically large value to the down mass allows to be more sensitive to the non-perturbative effects behind the $m_u=0$ ambiguity. Preliminary results are presented for four masses of clover fermions.