diff --git a/Cargo.lock b/Cargo.lock
index 4fb7b5a3..35f3e0e5 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1160,7 +1160,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_builtins"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "chrono",
  "fast-float",
@@ -1185,7 +1185,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_bytecode"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "num-bigint",
  "oxide_log",
@@ -1193,7 +1193,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_cfg"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "oxide_bytecode",
  "oxide_emit",
@@ -1204,7 +1204,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_cli"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "ansi_term",
  "bumpalo",
@@ -1228,7 +1228,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_code_cache"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "lru",
  "oxide_bytecode",
@@ -1237,7 +1237,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_compiler"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "oxide_bytecode",
  "oxide_cfg",
@@ -1258,7 +1258,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_dce"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "oxide_bytecode",
  "oxide_cfg",
@@ -1272,7 +1272,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_emit"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "num-bigint",
  "oxc_allocator",
@@ -1288,7 +1288,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_engine"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "oxide_bytecode",
  "oxide_compiler",
@@ -1300,7 +1300,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_fold"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "num-bigint",
  "oxide_bytecode",
@@ -1312,7 +1312,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_ir"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "oxide_bytecode",
  "oxide_log",
@@ -1321,7 +1321,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_kernel"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "bumpalo",
  "dashmap",
@@ -1335,7 +1335,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_liveness"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "oxide_bytecode",
  "oxide_cfg",
@@ -1347,7 +1347,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_log"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "tracing",
  "tracing-appender",
@@ -1356,11 +1356,11 @@ dependencies = [
 
 [[package]]
 name = "oxide_net"
-version = "0.19.8"
+version = "0.19.9"
 
 [[package]]
 name = "oxide_parser"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "oxc_allocator",
  "oxc_ast",
@@ -1375,7 +1375,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_regalloc"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "oxide_bytecode",
  "oxide_cfg",
@@ -1391,7 +1391,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_runtime_api"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "num-bigint",
  "num-traits",
@@ -1404,7 +1404,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_test262"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "oxide_bytecode",
  "oxide_compiler",
@@ -1424,7 +1424,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_types"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "bumpalo",
  "dashmap",
@@ -1437,7 +1437,7 @@ dependencies = [
 
 [[package]]
 name = "oxide_vm"
-version = "0.19.8"
+version = "0.19.9"
 dependencies = [
  "bumpalo",
  "chrono",
diff --git a/Cargo.toml b/Cargo.toml
index 15d12df6..4e024e5c 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -25,7 +25,7 @@ members = [
 ]
 
 [workspace.package]
-version = "0.19.8"
+version = "0.19.9"
 edition = "2021"
 license = "MIT"
 rust-version = "1.80"
diff --git a/crates/oxide_builtins/src/map.rs b/crates/oxide_builtins/src/map.rs
index 2ae25f4e..4bfcfe81 100644
--- a/crates/oxide_builtins/src/map.rs
+++ b/crates/oxide_builtins/src/map.rs
@@ -292,8 +292,8 @@ pub fn map_constructor<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
 pub fn map_set<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
     let this_val = vm.reg(if args.is_empty() { 0 } else { args[0] });
     let inner = native_try!(get_map_inner(vm, this_val));
-    let key = vm.reg(if args.len() > 1 { args[1] } else { 0 });
-    let val = vm.reg(if args.len() > 2 { args[2] } else { 0 });
+    let key = if args.len() > 1 { vm.reg(args[1]) } else { JsValue::undefined() };
+    let val = if args.len() > 2 { vm.reg(args[2]) } else { JsValue::undefined() };
     unsafe {
         (*inner).insert(SetKey(key), val);
     }
@@ -318,8 +318,8 @@ pub fn map_get<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
 pub fn map_get_or_insert<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
     let this_val = vm.reg(if args.is_empty() { 0 } else { args[0] });
     let inner = native_try!(get_map_inner(vm, this_val));
-    let key = vm.reg(if args.len() > 1 { args[1] } else { 0 });
-    let val = vm.reg(if args.len() > 2 { args[2] } else { 0 });
+    let key = if args.len() > 1 { vm.reg(args[1]) } else { JsValue::undefined() };
+    let val = if args.len() > 2 { vm.reg(args[2]) } else { JsValue::undefined() };
     let found = unsafe { (*inner).get(&SetKey(key)) };
     if let Some(existing) = found {
         return NativeResult::Ok(existing);
diff --git a/crates/oxide_vm/tests/builtins_map_tests.rs b/crates/oxide_vm/tests/builtins_map_tests.rs
index 9530b612..bb4bdd28 100644
--- a/crates/oxide_vm/tests/builtins_map_tests.rs
+++ b/crates/oxide_vm/tests/builtins_map_tests.rs
@@ -1,6 +1,9 @@
 use std::sync::Arc;
 
-use oxide_builtins::map::{map_clear, map_constructor as new_map, map_delete, map_get, map_has, map_set, map_size};
+use oxide_builtins::map::{
+    map_clear, map_constructor as new_map, map_delete, map_get, map_get_or_insert, map_has,
+    map_set, map_size,
+};
 use oxide_compiler::compiler::Compiler;
 use oxide_kernel::shape_forge::EMPTY_SHAPE_ID;
 use oxide_runtime_api::VmHost;
@@ -84,6 +87,31 @@ fn tmap_size_and_clear() {
     assert_eq!(map_size(&mut vm, &[0]).unwrap().as_double(), 0.0);
 }
 
+#[test]
+fn tmap_set_missing_value_defaults_undefined() {
+    // 缺省 value 实参：存储值取 undefined，不读调用方结果槽残留。
+    let mut vm = Vm::new();
+    map_this(&mut vm);
+    let m = new_map(&mut vm, &[0]).unwrap();
+    vm.set_reg(0, m);
+    vm.set_reg(1, JsValue::float(5.0));
+    map_set(&mut vm, &[0, 1]).unwrap();
+    assert!(map_get(&mut vm, &[0, 1]).unwrap().is_undefined());
+}
+
+#[test]
+fn tmap_get_or_insert_missing_value_defaults_undefined() {
+    // 缺省 value 实参且键缺失：返回值与存储值均取 undefined，不读调用方结果槽残留。
+    let mut vm = Vm::new();
+    map_this(&mut vm);
+    let m = new_map(&mut vm, &[0]).unwrap();
+    vm.set_reg(0, m);
+    vm.set_reg(1, JsValue::float(5.0));
+    let r = map_get_or_insert(&mut vm, &[0, 1]).unwrap();
+    assert!(r.is_undefined());
+    assert!(map_get(&mut vm, &[0, 1]).unwrap().is_undefined());
+}
+
 // ── 经 JS 使用 new 关键字的 eval 测试（每个测试一次 eval）──
 
 #[test]