diff --git a/opam b/opam
index a759d301f..193840cea 100644
--- a/opam
+++ b/opam
@@ -25,7 +25,7 @@ depends: [
   "atdgen" {>="2.0.0"}
   "base64" {>="3.0.0"}
   "cmdliner" {>="1.0.0"}
-  "core"
+  "core" {<"v0.12.0"}
   "conf-autoconf" {build}
   "conf-gmp" {build}
   "conf-mpfr" {build}