diff --git a/crates/backend/src/typegen/fn.rs b/crates/backend/src/typegen/fn.rs
index 4c93b3e8..48a779e5 100644
--- a/crates/backend/src/typegen/fn.rs
+++ b/crates/backend/src/typegen/fn.rs
@@ -1,5 +1,6 @@
 use convert_case::{Case, Casing};
 use quote::ToTokens;
+use syn::Pat;
 
 use super::{ty_to_ts_type, ToTypeDef, TypeDef};
 use crate::{CallbackArg, FnKind, NapiFn};
@@ -49,6 +50,11 @@ impl NapiFn {
           if path.ty.to_token_stream().to_string() == "Env" {
             return None;
           }
+          let mut path = path.clone();
+          // remove mutability from PatIdent
+          if let Pat::Ident(i) = path.pat.as_mut() {
+            i.mutability = None;
+          }
           let mut arg = path.pat.to_token_stream().to_string().to_case(Case::Camel);
           arg.push_str(": ");
           arg.push_str(&ty_to_ts_type(&path.ty, false));
diff --git a/examples/napi/__test__/typegen.spec.ts.md b/examples/napi/__test__/typegen.spec.ts.md
index 66ac07d0..a5582548 100644
--- a/examples/napi/__test__/typegen.spec.ts.md
+++ b/examples/napi/__test__/typegen.spec.ts.md
@@ -43,7 +43,7 @@ Generated by [AVA](https://avajs.dev).
     export function readPackageJson(): PackageJson␊
     export function getPackageJsonName(packageJson: PackageJson): string␊
     export function contains(source: string, target: string): boolean␊
-    export function concatStr(mutS: string): string␊
+    export function concatStr(s: string): string␊
     export function concatUtf16(s: string): string␊
     export function concatLatin1(s: string): string␊
     export function withoutAbortController(a: number, b: number): Promise<number>␊
diff --git a/examples/napi/__test__/typegen.spec.ts.snap b/examples/napi/__test__/typegen.spec.ts.snap
index 17956eb9..3b1d6568 100644
Binary files a/examples/napi/__test__/typegen.spec.ts.snap and b/examples/napi/__test__/typegen.spec.ts.snap differ
diff --git a/examples/napi/index.d.ts b/examples/napi/index.d.ts
index 66f9791c..a1e3c170 100644
--- a/examples/napi/index.d.ts
+++ b/examples/napi/index.d.ts
@@ -33,7 +33,7 @@ interface PackageJson {
 export function readPackageJson(): PackageJson
 export function getPackageJsonName(packageJson: PackageJson): string
 export function contains(source: string, target: string): boolean
-export function concatStr(mutS: string): string
+export function concatStr(s: string): string
 export function concatUtf16(s: string): string
 export function concatLatin1(s: string): string
 export function withoutAbortController(a: number, b: number): Promise<number>