diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 9392b7a..5c39f4d 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -24,10 +24,10 @@ jobs:
       with:
         environment-name: test-env
         create-args: >-
-          python
+          python=3.11
           pip
-          pandas
-          numpy
+          pandas=1
+          numpy=1
           xarray
           pytest
 
diff --git a/traittypes/tests/test_traittypes.py b/traittypes/tests/test_traittypes.py
index db3a80f..7b0e3a1 100644
--- a/traittypes/tests/test_traittypes.py
+++ b/traittypes/tests/test_traittypes.py
@@ -149,7 +149,7 @@ class TestSeries(TestCase):
     def test_series_equal(self):
         notifications = []
         class Foo(HasTraits):
-            bar = Series([1, 2])
+            bar = Series([1, 2], dtype=np.int64)
             @observe('bar')
             def _(self, change):
                 notifications.append(change)
diff --git a/traittypes/traittypes.py b/traittypes/traittypes.py
index 9de6e97..2ebf00a 100644
--- a/traittypes/traittypes.py
+++ b/traittypes/traittypes.py
@@ -155,18 +155,20 @@ def set(self, obj, value):
                 not old_value.equals(new_value)):
             obj._notify_trait(self.name, old_value, new_value)
 
-    def __init__(self, default_value=Empty, allow_none=False, klass=None, **kwargs):
+    def __init__(self, default_value=Empty, allow_none=False, klass=None, klass_kwargs=None, **kwargs):
         if klass is None:
             klass = self.klass
+        if klass_kwargs is None:
+            klass_kwargs = {}
         if (klass is not None) and inspect.isclass(klass):
             self.klass = klass
         else:
             raise TraitError('The klass attribute must be a class'
                                 ' not: %r' % klass)
         if default_value is Empty:
-            default_value = klass()
+            default_value = klass(**klass_kwargs)
         elif default_value is not None and default_value is not Undefined:
-            default_value = klass(default_value)
+            default_value = klass(default_value, **klass_kwargs)
         super(PandasType, self).__init__(default_value=default_value, allow_none=allow_none, **kwargs)
 
     def make_dynamic_default(self):
@@ -198,12 +200,12 @@ class Series(PandasType):
     info_text = 'a pandas series'
     dtype = None
 
-    def __init__(self, default_value=Empty, allow_none=False, dtype=None, **kwargs):
+    def __init__(self, default_value=Empty, allow_none=False, dtype=np.float64, **kwargs):
         if 'klass' not in kwargs and self.klass is None:
             import pandas as pd
             kwargs['klass'] = pd.Series
         super(Series, self).__init__(
-            default_value=default_value, allow_none=allow_none, **kwargs)
+            default_value=default_value, allow_none=allow_none, klass_kwargs={"dtype": dtype}, **kwargs)
         self.tag(dtype=dtype)
         self.dtype = dtype