diff --git a/src/crypto-secret/secret.cr b/src/crypto-secret/secret.cr
index 4e23287..082aee5 100644
--- a/src/crypto-secret/secret.cr
+++ b/src/crypto-secret/secret.cr
@@ -113,11 +113,11 @@ module Crypto::Secret
   # Makes a region allocated inaccessible depending on implementation. It cannot be read or written, but the data are preserved.
   abstract def noaccess : self
 
-  protected abstract def to_slice(& : Bytes -> Nil)
+  protected abstract def to_slice(& : Bytes -> U) forall U
   abstract def bytesize : Int32
 
   macro delegate_to_slice(to object)
-    def to_slice(& : Bytes -> Nil)
+    def to_slice(& : Bytes -> U) forall U
       yield {{object.id}}.to_slice
     end
   end
diff --git a/src/crypto-secret/stateful.cr b/src/crypto-secret/stateful.cr
index 98535a3..e22ac86 100644
--- a/src/crypto-secret/stateful.cr
+++ b/src/crypto-secret/stateful.cr
@@ -81,10 +81,12 @@ module Crypto::Secret
       return if @state == new_state
 
       case new_state
-      in State::Readwrite; readwrite
-      in State::Readonly ; readonly
-      in State::Noaccess ; noaccess
-      in State::Wiped    ; raise Error::KeyWiped.new
+      when State::Readwrite; readwrite
+      when State::Readonly ; readonly
+      when State::Noaccess ; noaccess
+      when State::Wiped    ; raise Error::KeyWiped.new
+      else
+        raise Error::InvalidStateTransition.new("can't change to #{new_state}")
       end
     end