diff --git a/builder-web.opam b/builder-web.opam
index 68cb62a..352d601 100644
--- a/builder-web.opam
+++ b/builder-web.opam
@@ -17,6 +17,7 @@ build: [
 depends: [
   "builder"
   "dream" {>= "1.0.0~alpha2"}
+  "cstruct" {>= "6.0.0"}
   "bos"
   "hex"
   "caqti"
diff --git a/db/representation.ml b/db/representation.ml
index 1fa626b..a4e6886 100644
--- a/db/representation.ml
+++ b/db/representation.ml
@@ -2,7 +2,7 @@ module Asn = struct
   let decode_strict codec cs =
     match Asn.decode codec cs with
     | Ok (a, cs) ->
-      if Cstruct.len cs = 0
+      if Cstruct.length cs = 0
       then Ok a
       else Error "trailing bytes"
     | Error (`Parse msg) -> Error ("parse error: " ^ msg)