diff --git a/src/interpreter/classes.rs b/src/interpreter/classes.rs
index 46c4bfc..d89facb 100644
--- a/src/interpreter/classes.rs
+++ b/src/interpreter/classes.rs
@@ -21,7 +21,7 @@ pub type ClassRef = Rc<RefCell<Class>>;
 #[derive(Debug, Clone)]
 pub struct Instance {
     class: Rc<RefCell<Class>>,
-    fields: HashMap<String, Value>,
+    fields: RefCell<HashMap<String, Value>>,
 }
 
 /* -------------------- *
@@ -76,12 +76,12 @@ impl Instance {
     fn new(class: ClassRef) -> Self {
         Self {
             class,
-            fields: HashMap::default(),
+            fields: RefCell::new(HashMap::default()),
         }
     }
 
     pub(super) fn get(&self, this_value: &Value, name: &Token) -> SloxResult<Value> {
-        if let Some(value) = self.fields.get(&name.lexeme) {
+        if let Some(value) = self.fields.borrow().get(&name.lexeme) {
             return Ok(value.clone());
         }
         if let Some(method) = self.class.borrow().methods.get(&name.lexeme) {
@@ -96,8 +96,8 @@ impl Instance {
         ))
     }
 
-    pub(super) fn set(&mut self, name: &Token, value: Value) {
-        self.fields.insert(name.lexeme.clone(), value);
+    pub(super) fn set(&self, name: &Token, value: Value) {
+        self.fields.borrow_mut().insert(name.lexeme.clone(), value);
     }
 
     fn bind_method(&self, method: &Function, this_value: &Value) -> Function {
diff --git a/src/interpreter/interpretable.rs b/src/interpreter/interpretable.rs
index 8b39932..adc500b 100644
--- a/src/interpreter/interpretable.rs
+++ b/src/interpreter/interpretable.rs
@@ -553,7 +553,7 @@ impl ExprNode {
         set_expr: &SetExpr,
     ) -> InterpreterResult {
         let instance = set_expr.instance.interpret(itpr_state)?.result();
-        instance.with_instance_mut(
+        instance.with_instance(
             |instance| {
                 let value = set_expr.value.interpret(itpr_state)?.result();
                 instance.set(&set_expr.name, value.clone());
diff --git a/src/interpreter/value.rs b/src/interpreter/value.rs
index 7f1b8cd..285170a 100644
--- a/src/interpreter/value.rs
+++ b/src/interpreter/value.rs
@@ -75,23 +75,6 @@ impl Value {
             _ => ferr(),
         }
     }
-
-    /// Run some code against a mutable instance value. If the value does
-    /// not contain an instance, an error function will be called instead.
-    pub fn with_instance_mut<Fok, Ferr, Rt>(&self, fok: Fok, ferr: Ferr) -> Rt
-    where
-        Fok: FnOnce(&mut Instance) -> Rt,
-        Ferr: FnOnce() -> Rt,
-    {
-        let mut obj = match self {
-            Value::Object(obj_ref) => obj_ref.borrow_mut(),
-            _ => return ferr(),
-        };
-        match &mut *obj {
-            Object::Instance(inst) => fok(inst),
-            _ => ferr(),
-        }
-    }
 }
 
 impl PartialEq for Value {