diff --git a/pandas/_libs/tslibs/timedeltas.pyx b/pandas/_libs/tslibs/timedeltas.pyx
index e1788db1cf8f8..77f5001443ae0 100644
--- a/pandas/_libs/tslibs/timedeltas.pyx
+++ b/pandas/_libs/tslibs/timedeltas.pyx
@@ -1181,7 +1181,7 @@ class Timedelta(_Timedelta):
 
             kwargs = {key: _to_py_int_float(kwargs[key]) for key in kwargs}
 
-            nano = kwargs.pop('nanoseconds', 0)
+            nano = np.timedelta64(kwargs.pop('nanoseconds', 0), 'ns')
             try:
                 value = nano + convert_to_timedelta64(timedelta(**kwargs),
                                                       'ns')
diff --git a/pandas/tests/arithmetic/test_datetime64.py b/pandas/tests/arithmetic/test_datetime64.py
index c81a371f37dc1..01108c1b8df03 100644
--- a/pandas/tests/arithmetic/test_datetime64.py
+++ b/pandas/tests/arithmetic/test_datetime64.py
@@ -2137,7 +2137,8 @@ def test_timedelta64_equal_timedelta_supported_ops(self, op):
         #                   'us': 1}
 
         def timedelta64(*args):
-            return sum(starmap(np.timedelta64, zip(args, intervals)))
+            # see casting notes in NumPy gh-12927
+            return np.sum(list(starmap(np.timedelta64, zip(args, intervals))))
 
         for d, h, m, s, us in product(*([range(2)] * 5)):
             nptd = timedelta64(d, h, m, s, us)