diff --git a/src/api.ml b/src/api.ml
index 058a2a59..28a4c4d6 100644
--- a/src/api.ml
+++ b/src/api.ml
@@ -105,7 +105,7 @@ module DenominationKey = struct
   let jsont =
     let open Jsont.Object in
     let rsa = Case.map "RSA" RsaDenominationKey.jsont ~dec:of_rsa in
-    let cs = Case.map "CS" Jsont.string ~dec:of_cs in
+    let cs = Case.map "CS" RsaDenominationKey.jsont ~dec:of_cs in
     let enc_case = function Rsa v -> Case.value rsa v in
     let cases = Case.[ make rsa; make cs ] in
     map ~kind:"DenominationKey" Fun.id