diff --git a/pyo3-macros-backend/src/params.rs b/pyo3-macros-backend/src/params.rs
index a6947b26..4e4607c2 100644
--- a/pyo3-macros-backend/src/params.rs
+++ b/pyo3-macros-backend/src/params.rs
@@ -4,6 +4,7 @@ use crate::{
     attributes::FromPyWithAttribute,
     method::{FnArg, FnSpec},
     pyfunction::Argument,
+    utils::unwrap_ty_group,
 };
 use proc_macro2::{Span, TokenStream};
 use quote::{quote, quote_spanned};
@@ -258,7 +259,7 @@ fn impl_arg_param(
         }
     };
 
-    return if let syn::Type::Reference(tref) = arg.optional.as_ref().unwrap_or(&ty) {
+    return if let syn::Type::Reference(tref) = unwrap_ty_group(arg.optional.unwrap_or(&ty)) {
         let (tref, mut_) = preprocess_tref(tref, self_);
         let (target_ty, borrow_tmp) = if arg.optional.is_some() {
             // Get Option<&T> from Option<PyRef<T>>
diff --git a/pyo3-macros-backend/src/utils.rs b/pyo3-macros-backend/src/utils.rs
index cc35c108..101a9b05 100644
--- a/pyo3-macros-backend/src/utils.rs
+++ b/pyo3-macros-backend/src/utils.rs
@@ -29,12 +29,8 @@ macro_rules! ensure_spanned {
 }
 
 /// Check if the given type `ty` is `pyo3::Python`.
-pub fn is_python(mut ty: &syn::Type) -> bool {
-    while let syn::Type::Group(group) = ty {
-        // Macros can create invisible delimiters around types.
-        ty = &*group.elem;
-    }
-    match ty {
+pub fn is_python(ty: &syn::Type) -> bool {
+    match unwrap_ty_group(ty) {
         syn::Type::Path(typath) => typath
             .path
             .segments
@@ -124,3 +120,10 @@ pub fn unwrap_group(mut expr: &syn::Expr) -> &syn::Expr {
     }
     expr
 }
+
+pub fn unwrap_ty_group(mut ty: &syn::Type) -> &syn::Type {
+    while let syn::Type::Group(g) = ty {
+        ty = &*g.elem;
+    }
+    ty
+}