diff --git a/codegen/src/size_measure/generator.rs b/codegen/src/size_measure/generator.rs
index 3fe6a6a..2e0a424 100644
--- a/codegen/src/size_measure/generator.rs
+++ b/codegen/src/size_measure/generator.rs
@@ -10,7 +10,7 @@ impl Generator {
     pub fn gen_impl(&self) -> TokenStream {
         let struct_ident = &self.struct_def.ident;
 
-        let mut num = -1;
+        let mut num = 0;
         let sum = self
             .struct_def
             .fields
@@ -21,9 +21,10 @@ impl Generator {
                         self.#i.aligned_size()
                     }
                 } else {
+                    let i = syn::Index::from(num);
                     num += 1;
                     quote! {
-                        self.#num.aligned_size()
+                        self.#i.aligned_size()
                     }
                 }
             })