diff --git a/opam b/opam
index a39f2257..10719d1a 100644
--- a/opam
+++ b/opam
@@ -6,7 +6,7 @@ maintainer: "simon.cruanes@inria.fr"
 build: [
     ["./configure"
         "--prefix" prefix
-        "--%{base-threads:enable%}-thread"
+        "--%{base-threads:enable}%-thread"
         "--disable-bench"
         "--disable-tests"
         "--%{lwt:enable}%-lwt"