diff --git a/opam/opam b/opam/opam
index a0d18ad0129448cb8df4f9f6c80d605a0acd2f29..d695a2b519994c3aefd5ad145ee454344922168c 100644
--- a/opam/opam
+++ b/opam/opam
@@ -126,8 +126,8 @@ depends: [
   "ocamlfind" # needed beyond build stage, used by -load-module
   "ocamlgraph" { >= "1.8.8" }
   "why3" { >= "1.5.0" }
-  "yojson" {>= "1.6.0"}
-  "zarith" {>= "1.5"}
+  "yojson" { >= "1.6.0" & < "2.0.0" }
+  "zarith" { >= "1.5" }
   "ppx_deriving"
   "ppx_import"
 ]