diff --git a/src/schema/script.rs b/src/schema/script.rs
index 603c2c2b..3d38e464 100644
--- a/src/schema/script.rs
+++ b/src/schema/script.rs
@@ -25,7 +25,7 @@
 //! Components related to the scripting system used by schema or applied at the
 //! specific contract operation level
 
-use std::ops::Deref;
+use std::ops::{Deref, DerefMut};
 
 use strict_types::TypeSystem;
 
@@ -101,6 +101,14 @@ impl Deref for Types {
     }
 }
 
+impl DerefMut for Types {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        match self {
+            Types::Strict(sys) => sys,
+        }
+    }
+}
+
 impl Default for Types {
     fn default() -> Self { Types::Strict(none!()) }
 }