diff --git a/src/signatures.ml b/src/signatures.ml
index 80cebad6..20d8a3e9 100644
--- a/src/signatures.ml
+++ b/src/signatures.ml
@@ -4,6 +4,9 @@
    check endianness
 
    better handling of decoding failure *)
+
+type _ t = R : 'r -> 'r t
+
 open Hash
 
 module Aliases = struct