diff --git a/derive/src/traits.rs b/derive/src/traits.rs
index 91296b8..dd425dc 100644
--- a/derive/src/traits.rs
+++ b/derive/src/traits.rs
@@ -210,17 +210,16 @@ fn generate_assert_no_padding(
   input: &DeriveInput,
 ) -> Result<TokenStream, &'static str> {
   let struct_type = &input.ident;
-  let span = input.span();
+  let span = input.ident.span();
   let fields = get_struct_fields(input)?;
 
   let field_types = get_field_types(&fields);
-  let struct_size =
-    quote_spanned!(span => ::core::mem::size_of::<#struct_type>());
   let size_sum =
     quote_spanned!(span => 0 #( + ::core::mem::size_of::<#field_types>() )*);
 
   Ok(quote_spanned! {span => const _: fn() = || {
-    let _ = ::core::mem::transmute::<[u8; #struct_size], [u8; #size_sum]>;
+    struct TypeWithoutPadding([u8; #size_sum]);
+    let _ = ::core::mem::transmute::<#struct_type, TypeWithoutPadding>;
   };})
 }