This note is a remark on Kock's work on linear algebra in the Zariski topos [2] . We point out that his main result implies a version of Cramer's rule for the generic local A-algebra in the Zariski topos Z/Spec(A) . A constructive version of the Jacobian criterion for unramified morphisms of [4] is obtained as a consequence. Furthermore, we prove a Jacobian criterion for 1-etale morphisms (introduced in [3] in the more general context of formal differential geometry). We assume some familiarity with the Kripke-Joyal semantics (cf. [2,6] ).