diff --git a/opam b/opam
index a0032544490c62e8179c7f090f3f05a2b090661c..5089492c9c3f4e26a334c6c1f12655d696456d86 100644
--- a/opam
+++ b/opam
@@ -135,7 +135,7 @@ depends: [
   "why3" { >= "1.6.0" & ( < "1.7.0" | !with-test ) }
   "yaml" { >= "3.0.0" }
   "yojson" {>= "1.6.0" & (>= "2.0.1" | !with-test)}
-  "zarith" { >= "1.5" }
+  "zarith" { >= "1.9" }
 
   # PPXs
   "ppx_deriving"