diff --git a/implementations/rust/Cargo.toml b/implementations/rust/Cargo.toml
index ca6e15a..d6b2b7c 100644
--- a/implementations/rust/Cargo.toml
+++ b/implementations/rust/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "preserves"
-version = "0.4.0"
+version = "0.5.0"
 authors = ["Tony Garnock-Jones <tonyg@leastfixedpoint.com>"]
 edition = "2018"
 description = "Implementation of the Preserves serialization format via serde."
diff --git a/implementations/rust/src/lib.rs b/implementations/rust/src/lib.rs
index 24c6790..67565ba 100644
--- a/implementations/rust/src/lib.rs
+++ b/implementations/rust/src/lib.rs
@@ -216,7 +216,11 @@ mod value_tests {
     #[test] fn record_mut() {
         let says = VV::symbol("says").wrap();
         let mut r = VV::record(says.clone(), vec![VV::from("Tony").wrap(), VV::from("Hello!").wrap()]);
-        r.as_record_mut().unwrap().1[0] = VV::from("Alice").wrap();
+        assert_eq!(r.as_record_mut(Some(0)), None);
+        assert_eq!(r.as_record_mut(Some(1)), None);
+        assert!(r.as_record_mut(Some(2)).is_some());
+        assert_eq!(r.as_record_mut(Some(3)), None);
+        r.as_record_mut(None).unwrap().1[0] = VV::from("Alice").wrap();
         assert_eq!(r, VV::record(says, vec![VV::from("Alice").wrap(), VV::from("Hello!").wrap()]));
     }
 
diff --git a/implementations/rust/src/value/de.rs b/implementations/rust/src/value/de.rs
index 22db7d9..79ed467 100644
--- a/implementations/rust/src/value/de.rs
+++ b/implementations/rust/src/value/de.rs
@@ -342,7 +342,7 @@ impl<'a, 'de, N: NestedValue<D>, D: 'de + Domain> EnumAccess<'de> for &'a mut De
     fn variant_seed<V>(self, seed: V)
                        -> Result<(V::Value, Self::Variant), D> where V: DeserializeSeed<'de>
     {
-        let (lp, _) = self.check(|v| v.as_record(), ExpectedKind::Record)?;
+        let (lp, _) = self.check(|v| v.as_record(None), ExpectedKind::Record(None))?;
         let v = self.input;
         self.input = N::boxunwrap(lp);
         let variant = seed.deserialize(&mut *self)?;
@@ -359,18 +359,18 @@ impl<'a, 'de, N: NestedValue<D>, D: 'de + Domain> VariantAccess<'de> for &'a mut
     }
 
     fn newtype_variant_seed<T>(self, seed: T) -> Result<T::Value, D> where T: DeserializeSeed<'de> {
-        let (_, fs) = self.input.value().as_record().unwrap();
+        let (_, fs) = self.check(|v| v.as_record(Some(1)), ExpectedKind::Record(Some(1)))?;
         self.input = &fs[0];
         seed.deserialize(&mut *self)
     }
 
     fn tuple_variant<V>(self, _len: usize, visitor: V) -> Result<V::Value, D> where V: Visitor<'de> {
-        visitor.visit_seq(VecSeq::new(self, &self.input.value().as_record().unwrap().1))
+        visitor.visit_seq(VecSeq::new(self, &self.input.value().as_record(None).unwrap().1))
     }
 
-    fn struct_variant<V>(self, _fields: &'static [&'static str], visitor: V)
+    fn struct_variant<V>(self, fields: &'static [&'static str], visitor: V)
                          -> Result<V::Value, D> where V: Visitor<'de>
     {
-        visitor.visit_seq(VecSeq::new(self, &self.input.value().as_record().unwrap().1))
+        visitor.visit_seq(VecSeq::new(self, &self.input.value().as_record(Some(fields.len())).unwrap().1))
     }
 }
diff --git a/implementations/rust/src/value/error.rs b/implementations/rust/src/value/error.rs
index f13a67e..188a969 100644
--- a/implementations/rust/src/value/error.rs
+++ b/implementations/rust/src/value/error.rs
@@ -22,7 +22,7 @@ pub enum ExpectedKind {
     ByteString,
     Symbol,
 
-    Record,
+    Record(Option<usize>),
     SimpleRecord(&'static str, Option<usize>),
     Option,
     Sequence,
diff --git a/implementations/rust/src/value/value.rs b/implementations/rust/src/value/value.rs
index 5760486..c228f6c 100644
--- a/implementations/rust/src/value/value.rs
+++ b/implementations/rust/src/value/value.rs
@@ -400,20 +400,28 @@ impl<N: NestedValue<D>, D: Domain> Value<N, D> {
     }
 
     pub fn is_record(&self) -> bool {
-        self.as_record().is_some()
+        self.as_record(None).is_some()
     }
 
-    pub fn as_record(&self) -> Option<&Record<N, D>> {
+    pub fn as_record(&self, arity: Option<usize>) -> Option<&Record<N, D>> {
         if let Value::Record(ref r) = *self {
-            Some(r)
+            match arity {
+                Some(expected) if r.1.len() == expected => Some(r),
+                Some(_other) => None,
+                None => Some(r)
+            }
         } else {
             None
         }
     }
 
-    pub fn as_record_mut(&mut self) -> Option<&mut Record<N, D>> {
+    pub fn as_record_mut(&mut self, arity: Option<usize>) -> Option<&mut Record<N, D>> {
         if let Value::Record(ref mut r) = *self {
-            Some(r)
+            match arity {
+                Some(expected) if r.1.len() == expected => Some(r),
+                Some(_other) => None,
+                None => Some(r)
+            }
         } else {
             None
         }
@@ -428,14 +436,9 @@ impl<N: NestedValue<D>, D: Domain> Value<N, D> {
     }
 
     pub fn as_simple_record(&self, label: &str, arity: Option<usize>) -> Option<&Vec<N>> {
-        self.as_record().and_then(|(lp,fs)| {
+        self.as_record(arity).and_then(|(lp,fs)| {
             match N::boxunwrap(lp).value() {
-                Value::Symbol(ref s) if s == label =>
-                    match arity {
-                        Some(expected) if fs.len() == expected => Some(fs),
-                        Some(_other) => None,
-                        None => Some(fs)
-                    }
+                Value::Symbol(ref s) if s == label => Some(fs),
                 _ => None
             }
         })