diff --git a/crates/oxc_ast/src/ast_impl/ts.rs b/crates/oxc_ast/src/ast_impl/ts.rs
index c9d13e98e9aeb..624ccbcaa9037 100644
--- a/crates/oxc_ast/src/ast_impl/ts.rs
+++ b/crates/oxc_ast/src/ast_impl/ts.rs
@@ -55,7 +55,9 @@ impl<'a> TSType<'a> {
     /// Check if type maybe `undefined`
     pub fn is_maybe_undefined(&self) -> bool {
         match self {
-            TSType::TSUndefinedKeyword(_) => true,
+            TSType::TSAnyKeyword(_)
+            | TSType::TSUnknownKeyword(_)
+            | TSType::TSUndefinedKeyword(_) => true,
             TSType::TSUnionType(un) => un.types.iter().any(Self::is_maybe_undefined),
             _ => false,
         }
diff --git a/crates/oxc_isolated_declarations/src/function.rs b/crates/oxc_isolated_declarations/src/function.rs
index d6960f691e560..743f2fccefe41 100644
--- a/crates/oxc_isolated_declarations/src/function.rs
+++ b/crates/oxc_isolated_declarations/src/function.rs
@@ -95,7 +95,7 @@ impl<'a> IsolatedDeclarations<'a> {
                         if matches!(ts_type, TSType::TSTypeReference(_)) {
                             self.error(implicitly_adding_undefined_to_type(param.span));
                         } else if !ts_type.is_maybe_undefined() {
-                            // union with undefined
+                            // union with `undefined`
                             return self.ast.ts_type_annotation(
                                 SPAN,
                                 self.ast.ts_type_union_type(
diff --git a/crates/oxc_isolated_declarations/tests/fixtures/function-parameters.ts b/crates/oxc_isolated_declarations/tests/fixtures/function-parameters.ts
index e22af6aac55b9..4e9efc68b9d19 100644
--- a/crates/oxc_isolated_declarations/tests/fixtures/function-parameters.ts
+++ b/crates/oxc_isolated_declarations/tests/fixtures/function-parameters.ts
@@ -26,3 +26,7 @@ export function fooBad([a, b] = [1, 2]): number {
 export const fooBad2 = ({a, b} = { a: 1, b: 2 }): number => {
   return 2;
 }
+
+
+export function withAny(a: any = 1, b: string): void { }
+export function withUnknown(a: unknown = 1, b: string): void { }
diff --git a/crates/oxc_isolated_declarations/tests/snapshots/function-parameters.snap b/crates/oxc_isolated_declarations/tests/snapshots/function-parameters.snap
index c006d77d4c613..458de230d6d28 100644
--- a/crates/oxc_isolated_declarations/tests/snapshots/function-parameters.snap
+++ b/crates/oxc_isolated_declarations/tests/snapshots/function-parameters.snap
@@ -14,6 +14,8 @@ export declare function fnDeclBad2<T>(p: T, r2: T): void;
 export declare function fnDeclBad3<T>(p: T, rParam?: T, r2: T): void;
 export declare function fooBad(): number;
 export declare const fooBad2: () => number;
+export declare function withAny(a: any, b: string): void;
+export declare function withUnknown(a: unknown, b: string): void;
 
 
 ==================== Errors ====================