diff --git a/sedlex.opam b/sedlex.opam
index de2fe5d5..b77934d9 100644
--- a/sedlex.opam
+++ b/sedlex.opam
@@ -31,3 +31,6 @@ depends: [
   "gen"
   "uchar"
 ]
+conflicts: [
+  "wcs-lib" {= "2017-05-26.03" | = "2017-05-26.04"}
+]