diff --git a/crates/oxide_builtins/src/object.rs b/crates/oxide_builtins/src/object.rs
index 064e8df..4284fc2 100644
--- a/crates/oxide_builtins/src/object.rs
+++ b/crates/oxide_builtins/src/object.rs
@@ -140,8 +140,11 @@ pub fn object_assign<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
 }
 
 pub fn object_is<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
-    let lhs = if args.len() > 1 { vm.reg(args[1]) } else { JsValue::undefined() };
-    let rhs = if args.len() > 2 { vm.reg(args[2]) } else { JsValue::undefined() };
+    if args.len() < 3 {
+        return NativeResult::Err(crate::error::create_type_error(vm, "Object.is called with insufficient arguments"));
+    }
+    let lhs = vm.reg(args[1]);
+    let rhs = vm.reg(args[2]);
     NativeResult::Ok(JsValue::bool(oxide_runtime_api::same_value(lhs, rhs)))
 }
 
@@ -230,7 +233,10 @@ pub fn object_define_property<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResul
 
 pub fn object_get_own_property_descriptor<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
     if args.len() < 3 {
-        return NativeResult::Err(JsValue::undefined());
+        return NativeResult::Err(crate::error::create_type_error(
+            vm,
+            "Object.getOwnPropertyDescriptor called on non-object",
+        ));
     }
     let obj_val = vm.reg(args[1]);
     if !obj_val.is_object() {
diff --git a/crates/oxide_builtins/tests/object_boundary_tests.rs b/crates/oxide_builtins/tests/object_boundary_tests.rs
index 9475bb9..c8f3a80 100644
--- a/crates/oxide_builtins/tests/object_boundary_tests.rs
+++ b/crates/oxide_builtins/tests/object_boundary_tests.rs
@@ -196,3 +196,43 @@ fn test_from_entries_empty_array() {
     let (_vm, result) = eval("Object.fromEntries([])").unwrap();
     assert!(result.is_object(), "fromEntries([]) should return an empty object");
 }
+
+#[test]
+fn test_object_is_nan() {
+    let (_vm, result) = eval("Object.is(NaN, NaN)").unwrap();
+    assert!(result.is_bool() && result.as_bool());
+}
+
+#[test]
+fn test_object_is_signed_zero() {
+    let (_vm, result) = eval("Object.is(0, -0)").unwrap();
+    assert!(result.is_bool() && !result.as_bool());
+}
+
+#[test]
+fn test_entries_returns_key_value_pairs() {
+    let (_vm, result) = eval("var o = {a:1, b:2}; Object.entries(o).length").unwrap();
+    assert!(result.is_int() || result.is_double());
+}
+
+#[test]
+fn test_values_returns_array() {
+    let (_vm, result) = eval("var o = {a:1, b:2}; Object.values(o).length").unwrap();
+    assert!(result.is_int() || result.is_double());
+}
+
+#[test]
+fn test_get_own_property_descriptor_no_args_type_error() {
+    assert!(eval("Object.getOwnPropertyDescriptor()").is_err());
+}
+
+#[test]
+fn test_has_own_property_non_object_this_type_error() {
+    assert!(eval("Object.prototype.hasOwnProperty.call(42, 'x')").is_err());
+}
+
+#[test]
+fn test_property_is_enumerable_own_property() {
+    let (_vm, result) = eval("var o = {x: 1}; o.propertyIsEnumerable('x')").unwrap();
+    assert!(result.is_bool() && result.as_bool());
+}