diff --git a/vm/src/objtype.rs b/vm/src/objtype.rs
index 2185c26b1..c429e0e26 100644
--- a/vm/src/objtype.rs
+++ b/vm/src/objtype.rs
@@ -1,4 +1,5 @@
 use super::objdict;
+use super::objtype; // Required for arg_check! to use isinstance
 use super::pyobject::{
     AttributeProtocol, IdProtocol, PyContext, PyFuncArgs, PyObject, PyObjectKind, PyObjectRef,
     PyResult, ToRust, TypeProtocol,
@@ -28,7 +29,16 @@ pub fn init(context: &PyContext) {
 }
 
 fn type_mro(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
-    match _mro(args.args[0].clone()) {
+    println!("{:?}", args);
+    arg_check!(
+        vm,
+        args,
+        required = [
+            (cls, Some(vm.ctx.type_type())),
+            (_typ, Some(vm.ctx.type_type()))
+        ]
+    );
+    match _mro(cls.clone()) {
         Some(mro) => Ok(vm.context().new_tuple(mro)),
         None => Err(vm.new_type_error("Only classes have an MRO.".to_string())),
     }
@@ -71,14 +81,29 @@ pub fn get_type_name(typ: &PyObjectRef) -> String {
 pub fn type_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     debug!("type.__new__{:?}", args);
     if args.args.len() == 2 {
-        Ok(args.args[1].typ())
+        arg_check!(
+            vm,
+            args,
+            required = [(_typ, Some(vm.ctx.type_type())), (obj, None)]
+        );
+        Ok(obj.typ())
     } else if args.args.len() == 4 {
-        let typ = args.args[0].clone();
-        let name = args.args[1].to_str().unwrap();
+        arg_check!(
+            vm,
+            args,
+            required = [
+                (typ, Some(vm.ctx.type_type())),
+                (name, Some(vm.ctx.str_type())),
+                // bases needs to be mutable, which arg_check! doesn't support, so we just check
+                // the type and extract it again below
+                // TODO: arg_check! should support specifying iterables
+                (_bases, None),
+                (dict, Some(vm.ctx.dict_type()))
+            ]
+        );
         let mut bases = args.args[2].to_vec().unwrap();
         bases.push(vm.context().object());
-        let dict = args.args[3].clone();
-        new(typ, &name, bases, dict)
+        new(typ.clone(), &name.to_str().unwrap(), bases, dict.clone())
     } else {
         Err(vm.new_type_error(format!(": type_new: {:?}", args)))
     }
@@ -185,9 +210,7 @@ pub fn new(typ: PyObjectRef, name: &str, bases: Vec<PyObjectRef>, dict: PyObject
 }
 
 fn type_str(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
-    // TODO: fix macro:
-    // arg_check!(vm, args, required = [(obj, Some(vm.ctx.type_type()))]);
-    let obj = args.args[0].clone();
+    arg_check!(vm, args, required = [(obj, Some(vm.ctx.type_type()))]);
     let type_name = get_type_name(&obj);
     Ok(vm.new_str(format!("<class '{}'>", type_name)))
 }