diff --git a/crates/oxide_builtins/src/date.rs b/crates/oxide_builtins/src/date.rs
index c723495..91fa984 100644
--- a/crates/oxide_builtins/src/date.rs
+++ b/crates/oxide_builtins/src/date.rs
@@ -734,6 +734,14 @@ pub fn date_get_year<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
 pub fn date_set_year<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
     let obj = unsafe { &mut *native_try!(date_this_mut(vm, args)) };
     let ms = get_timestamp(obj);
+    // A missing year argument coerces to NaN; per B.2.4.2 a NaN year sets the date value to
+    // NaN and returns NaN.
+    let arg = if args.len() > 1 { vm.reg(args[1]) } else { JsValue::undefined() };
+    let y_num = oxide_runtime_api::to_number(arg);
+    if y_num.is_nan() {
+        set_timestamp(obj, f64::NAN);
+        return NativeResult::Ok(JsValue::float(f64::NAN));
+    }
     if !ms.is_finite() {
         return NativeResult::Ok(JsValue::float(f64::NAN));
     }
@@ -741,7 +749,7 @@ pub fn date_set_year<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
         Some(d) => d,
         None => return NativeResult::Ok(JsValue::float(f64::NAN)),
     };
-    let y = oxide_runtime_api::to_number(vm.reg(args[1])).trunc() as i32;
+    let y = y_num.trunc() as i32;
     let full_year = if (0..=99).contains(&y) { y + 1900 } else { y };
     let nd = dt.with_year(full_year).unwrap_or(dt);
     let ts = nd.timestamp_millis() as f64;
diff --git a/crates/oxide_vm/tests/builtins_date_tests.rs b/crates/oxide_vm/tests/builtins_date_tests.rs
index aa3927e..ad29001 100644
--- a/crates/oxide_vm/tests/builtins_date_tests.rs
+++ b/crates/oxide_vm/tests/builtins_date_tests.rs
@@ -345,3 +345,40 @@ fn date_set_utc_invalid_returns_nan() {
     let r = eval(&mut vm, "new Date(NaN).setUTCFullYear(2025)").unwrap();
     assert!(r.as_double().is_nan());
 }
+
+// setYear with no argument must not index a missing argument register (previously panicked);
+// a missing / NaN year sets the date value to NaN and returns NaN (B.2.4.2).
+#[test]
+fn date_set_year_no_arg_returns_nan() {
+    let mut vm = Vm::new();
+    let r = eval(&mut vm, "var d = new Date(0); d.setYear()").unwrap();
+    assert!(r.as_double().is_nan());
+}
+
+#[test]
+fn date_set_year_no_arg_sets_value_nan() {
+    let mut vm = Vm::new();
+    let r = eval(&mut vm, "var d = new Date(0); d.setYear(); d.valueOf()").unwrap();
+    assert!(r.as_double().is_nan());
+}
+
+#[test]
+fn date_set_year_nan_from_string() {
+    let mut vm = Vm::new();
+    let r = eval(&mut vm, "var d = new Date(0); d.setYear('not a number')").unwrap();
+    assert!(r.as_double().is_nan());
+}
+
+#[test]
+fn date_set_year_two_digit_maps_to_1900s() {
+    let mut vm = Vm::new();
+    let r = eval(&mut vm, "var d = new Date(0); d.setYear(95); d.getFullYear()").unwrap();
+    assert_eq!(r.as_double() as i64, 1995);
+}
+
+#[test]
+fn date_set_year_four_digit_kept() {
+    let mut vm = Vm::new();
+    let r = eval(&mut vm, "var d = new Date(0); d.setYear(2020); d.getFullYear()").unwrap();
+    assert_eq!(r.as_double() as i64, 2020);
+}