diff --git a/crates/oxide_vm/src/dispatch/property.rs b/crates/oxide_vm/src/dispatch/property.rs
index 7cb78a4..cad2ad7 100644
--- a/crates/oxide_vm/src/dispatch/property.rs
+++ b/crates/oxide_vm/src/dispatch/property.rs
@@ -98,6 +98,11 @@ impl Vm {
         let prop_name_si = self.property_key_si(self.regs[b]);
         if self.kernel.string_forge().lookup(prop_name_si).as_deref() == Some("__proto__") {
             let obj = unsafe { &mut *obj_ptr };
+            if self.is_object_prototype(obj_ptr) && !self.regs[a].is_null() {
+                self.raise_type_error("Object.prototype.__proto__ is immutable")?;
+                self.pc += 3;
+                return Ok(());
+            }
             obj.set_proto(self.regs[a]).map_err(|e| e.to_string())?;
             self.pc += 3;
             return Ok(());
@@ -171,6 +176,10 @@ impl Vm {
         let obj = unsafe { &mut *obj_ptr };
         let prop_name_si = self.property_key_si(self.regs[b]);
         if self.kernel.string_forge().lookup(prop_name_si).as_deref() == Some("__proto__") {
+            if self.is_object_prototype(obj_ptr) && !self.regs[a].is_null() {
+                self.raise_type_error("Object.prototype.__proto__ is immutable")?;
+                return Ok(());
+            }
             obj.set_proto(self.regs[a]).map_err(|e| e.to_string())?;
             return Ok(());
         }
@@ -199,6 +208,10 @@ impl Vm {
         let obj = unsafe { &mut *obj_ptr };
         let prop_name_si = self.property_key_si(self.regs[a]);
         if self.kernel.string_forge().lookup(prop_name_si).as_deref() == Some("__proto__") {
+            if self.is_object_prototype(obj_ptr) && !self.regs[b].is_null() {
+                self.raise_type_error("Object.prototype.__proto__ is immutable")?;
+                return Ok(());
+            }
             obj.set_proto(self.regs[b]).map_err(|e| e.to_string())?;
             return Ok(());
         }
diff --git a/crates/oxide_vm/src/vm.rs b/crates/oxide_vm/src/vm.rs
index 8a6e8ad..a6f13f9 100644
--- a/crates/oxide_vm/src/vm.rs
+++ b/crates/oxide_vm/src/vm.rs
@@ -198,6 +198,11 @@ impl Vm {
         &self.kernel
     }
 
+    pub(crate) fn is_object_prototype(&self, ptr: *const JsObject) -> bool {
+        let proto_ptr = self.kernel.builtin_world().object_proto.as_ptr();
+        std::ptr::eq(ptr, proto_ptr)
+    }
+
     pub fn reg(&self, idx: u8) -> JsValue {
         self.regs[idx as usize]
     }