diff --git a/codegen/src/custom_type.rs b/codegen/src/custom_type.rs
index fa182fa02..8b08af1fb 100644
--- a/codegen/src/custom_type.rs
+++ b/codegen/src/custom_type.rs
@@ -48,8 +48,8 @@ pub fn derive_custom_type_impl(input: DeriveInput) -> TokenStream {
 
     quote! {
         impl ::rhai::CustomType for #name {
-            fn build(builder: ::rhai::TypeBuilder<'_, Self>) {
-                #accessors
+            fn build(mut builder: ::rhai::TypeBuilder<'_, Self>) {
+                #accessors;
             }
         }
     }
@@ -63,11 +63,11 @@ fn generate_accessor_fns(
 ) -> proc_macro2::TokenStream {
     let get = get
         .map(|func| quote! {#func})
-        .unwrap_or_else(|| quote! {|obj| obj.#field.clone()});
+        .unwrap_or_else(|| quote! {|obj: &mut Self| obj.#field.clone()});
 
     let set = set
         .map(|func| quote! {#func})
-        .unwrap_or_else(|| quote! {|obj, val| obj.#field = val});
+        .unwrap_or_else(|| quote! {|obj: &mut Self, val| obj.#field = val});
 
     if readonly {
         quote! {
diff --git a/codegen/tests/test_derive.rs b/codegen/tests/test_derive.rs
new file mode 100644
index 000000000..c24938284
--- /dev/null
+++ b/codegen/tests/test_derive.rs
@@ -0,0 +1,15 @@
+use rhai_codegen::CustomType;
+
+// Sanity check to make sure everything compiles
+#[derive(Clone, CustomType)]
+pub struct Foo {
+    #[get(get_bar)]
+    bar: i32,
+    #[readonly]
+    baz: String,
+    qux: Vec<i32>,
+}
+
+fn get_bar(_this: &mut Foo) -> i32 {
+    42
+}