diff --git a/implementations/rust/src/lib.rs b/implementations/rust/src/lib.rs
index bba7940..635680b 100644
--- a/implementations/rust/src/lib.rs
+++ b/implementations/rust/src/lib.rs
@@ -22,11 +22,15 @@ mod dom {
     }
 
     impl Domain for Dom {
-        fn as_preserves(&self) -> Result<IOValue, std::io::Error> {
-            Ok(match self {
+        fn from_preserves(v: IOValue) -> Self {
+            panic!("Cannot decode IOValue to Dom: {:?}", v);
+        }
+
+        fn as_preserves(&self) -> IOValue {
+            match self {
                 Dom::One => Value::ByteString(vec![255, 255, 255, 255]).wrap(),
                 Dom::Two => Value::symbol(&format!("Dom::{:?}", self)).wrap(),
-            })
+            }
         }
     }
 
diff --git a/implementations/rust/src/value/packed/reader.rs b/implementations/rust/src/value/packed/reader.rs
index e838381..3da0cd4 100644
--- a/implementations/rust/src/value/packed/reader.rs
+++ b/implementations/rust/src/value/packed/reader.rs
@@ -264,7 +264,7 @@ impl<'de, S: BinarySource<'de>> Reader<'de> for PackedReader<'de, S> {
             }
             Tag::Pointer => {
                 let v = self.demand_next(read_annotations)?;
-                Value::Domain(IOValue::from_preserves(v)?).wrap()
+                Value::Domain(IOValue::from_preserves(v)).wrap()
             }
             Tag::SmallInteger(v) => {
                 // TODO: prebuild these in value.rs
diff --git a/implementations/rust/src/value/value.rs b/implementations/rust/src/value/value.rs
index 32213e0..c0613b7 100644
--- a/implementations/rust/src/value/value.rs
+++ b/implementations/rust/src/value/value.rs
@@ -19,17 +19,8 @@ use super::signed_integer::SignedInteger;
 use crate::error::{Error, ExpectedKind, Received};
 
 pub trait Domain: Sized + Debug + Clone + Eq + Hash + Ord {
-    fn from_preserves(v: IOValue) -> Result<Self, std::io::Error> {
-        Err(std::io::Error::new(std::io::ErrorKind::InvalidData,
-                                format!("Cannot Preserves-decode domain-specific value {:?}",
-                                        v)))
-    }
-
-    fn as_preserves(&self) -> Result<IOValue, std::io::Error> {
-        Err(std::io::Error::new(std::io::ErrorKind::InvalidData,
-                                format!("Cannot Preserves-encode domain-specific value {:?}",
-                                        self)))
-    }
+    fn from_preserves(v: IOValue) -> Self;
+    fn as_preserves(&self) -> IOValue;
 }
 
 pub trait NestedValue<D: Domain>: Sized + Debug + Clone + Eq + Hash + Ord {
@@ -55,11 +46,11 @@ pub trait NestedValue<D: Domain>: Sized + Debug + Clone + Eq + Hash + Ord {
     }
 
     fn to_io_value(&self) -> IOValue {
-        self.copy_via(&|d| d.as_preserves().unwrap().value().clone())
+        self.copy_via(&|d| d.as_preserves().value().clone())
     }
 
     fn from_io_value(v: &IOValue) -> Self {
-        v.copy_via(&|_| unreachable!())
+        v.copy_via(&|d| Value::Domain(D::from_preserves(d.clone())))
     }
 }
 
@@ -1055,12 +1046,12 @@ pub struct IOValue(Arc<AnnotatedValue<IOValue, IOValue>>);
 pub type UnwrappedIOValue = Value<IOValue, IOValue>;
 
 impl Domain for IOValue {
-    fn from_preserves(v: IOValue) -> Result<Self, std::io::Error> {
-        Ok(v)
+    fn from_preserves(v: IOValue) -> Self {
+        v
     }
 
-    fn as_preserves(&self) -> Result<IOValue, std::io::Error> {
-        Ok(self.clone())
+    fn as_preserves(&self) -> IOValue {
+        self.clone()
     }
 }
 
diff --git a/implementations/rust/src/value/writer.rs b/implementations/rust/src/value/writer.rs
index f74dd43..5a05560 100644
--- a/implementations/rust/src/value/writer.rs
+++ b/implementations/rust/src/value/writer.rs
@@ -135,7 +135,7 @@ pub trait Writer: Sized {
             }
             Value::Domain(ref d) => {
                 let mut c = self.start_pointer()?;
-                c.write(&d.as_preserves()?)?;
+                c.write(&d.as_preserves())?;
                 self.end_pointer(c)
             }
         }