diff --git a/py/runtime.c b/py/runtime.c
index 2c3b3ddde4..ea3553db72 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -814,7 +814,7 @@ void mp_call_prepare_args_n_kw_var(bool have_self, size_t n_args_n_kw, const mp_
                     mp_obj_t iterable = mp_getiter(arg, &iter_buf);
                     mp_obj_t item;
                     while ((item = mp_iternext(iterable)) != MP_OBJ_STOP_ITERATION) {
-                        if (args2_len >= args2_alloc) {
+                        if (args2_len + (n_args - i) >= args2_alloc) {
                             args2 = mp_nonlocal_realloc(args2, args2_alloc * sizeof(mp_obj_t),
                                 args2_alloc * 2 * sizeof(mp_obj_t));
                             args2_alloc *= 2;
diff --git a/tests/basics/fun_callstardblstar.py b/tests/basics/fun_callstardblstar.py
index f395df3333..c08e46f668 100644
--- a/tests/basics/fun_callstardblstar.py
+++ b/tests/basics/fun_callstardblstar.py
@@ -37,3 +37,7 @@ f2(*iter(range(4)), **{'a': 1})
 
 # case where *args is not a tuple/list and takes up most of the memory allocated for **kwargs
 f2(*iter(range(100)), **{str(i): i for i in range(100)})
+
+# regression test - iterable with unknown len() was exactly using preallocated
+# memory causing args 4 and 5 to overflow the allocated arg array
+print(1, *iter((1, 2, 3)), *iter((1, 2, 3)), 4, 5, sep=",")