diff --git a/numbergen/__init__.py b/numbergen/__init__.py
index 6b356586c..ee1dfbf74 100644
--- a/numbergen/__init__.py
+++ b/numbergen/__init__.py
@@ -320,7 +320,7 @@ class TimeAwareRandomState(TimeAware):
     random_generator = param.Parameter(
         default=random.Random(c_size_t(hash((500,500))).value), doc=
         """
-        Random state used by the object. This may may be an instance
+        Random state used by the object. This may be an instance
         of random.Random from the Python standard library or an
         instance of numpy.random.RandomState.
 
@@ -458,9 +458,9 @@ class UniformRandom(RandomDistribution):
     See the random module for further details.
     """
 
-    lbound = param.Number(default=0.0,doc="inclusive lower bound")
+    lbound = param.Number(default=0.0,doc="Inclusive lower bound.")
 
-    ubound = param.Number(default=1.0,doc="exclusive upper bound")
+    ubound = param.Number(default=1.0,doc="Exclusive upper bound.")
 
 
     def __call__(self):
@@ -500,8 +500,8 @@ class UniformRandomInt(RandomDistribution):
     See the randint function in the random module for further details.
     """
 
-    lbound = param.Number(default=0,doc="inclusive lower bound")
-    ubound = param.Number(default=1000,doc="inclusive upper bound")
+    lbound = param.Number(default=0,doc="Inclusive lower bound.")
+    ubound = param.Number(default=1000,doc="Inclusive upper bound.")
 
 
     def __call__(self):
diff --git a/param/__init__.py b/param/__init__.py
index 748cce7ed..ffd95d5b3 100644
--- a/param/__init__.py
+++ b/param/__init__.py
@@ -1666,7 +1666,7 @@ def sig(self):
 
 
 class _SignatureSelector(Parameter):
-
+    # Needs docstring; why is this a separate mixin?
     _slot_defaults = _dict_update(
         SelectorBase._slot_defaults, _objects=_compute_selector_default,
         compute_default_fn=None, check_on_set=_compute_selector_checking_default,
@@ -1748,17 +1748,13 @@ def __init__(self, *, objects=Undefined, default=Undefined, instantiate=Undefine
             self.allow_None = self._slot_defaults['allow_None']
         else:
             self.allow_None = allow_None
-        if self.default is not None and self.check_on_set is True:
-            self._validate(self.default)
+        if self.default is not None:
+            self._validate_value(self.default)
         self._update_state()
 
     def _update_state(self):
-        if (
-            self.check_on_set is False
-            and self.default is not None
-            and self.default not in self.objects
-        ):
-            self.objects.append(self.default)
+        if self.check_on_set is False and self.default is not None:
+            self._ensure_value_is_in_objects(self.default)
 
     @property
     def objects(self):
@@ -1786,18 +1782,17 @@ def compute_default(self):
         """
         if self.default is None and callable(self.compute_default_fn):
             self.default = self.compute_default_fn()
-            if self.default not in self.objects:
-                self.objects.append(self.default)
+            self._ensure_value_is_in_objects(self.default)
 
     def _validate(self, val):
-        """
-        val must be None or one of the objects in self.objects.
-        """
         if not self.check_on_set:
             self._ensure_value_is_in_objects(val)
             return
 
-        if not (val in self.objects or (self.allow_None and val is None)):
+        self._validate_value(val)
+
+    def _validate_value(self, val):
+        if self.check_on_set and not (self.allow_None and val is None) and val not in self.objects:
             items = []
             limiter = ']'
             length = 0
@@ -2623,16 +2618,34 @@ def compute_default(self):
                     self.objects.append(o)
 
     def _validate(self, val):
+        self._validate_type(val)
+
+        if self.check_on_set:
+            self._validate_value(val)
+        else:
+            self._ensure_value_is_in_objects(val)
+
+
+    def _validate_type(self, val):
         if (val is None and self.allow_None):
             return
+
         if not isinstance(val, list):
             raise ValueError(
                 f"{_validate_error_prefix(self)} only takes list types, "
                 f"not {val!r}."
             )
-        for o in val:
-            super()._validate(o)
 
+    def _validate_value(self, val):
+        self._validate_type(val)
+        if val is not None:
+            for o in val:
+                super()._validate_value(o)
+
+    def _update_state(self):
+        if self.check_on_set is False and self.default is not None:
+            for o in self.default:
+                self._ensure_value_is_in_objects(o)
 
 
 class MultiFileSelector(ListSelector):
diff --git a/tests/testlistselector.py b/tests/testlistselector.py
index c737333c4..e61e052d5 100644
--- a/tests/testlistselector.py
+++ b/tests/testlistselector.py
@@ -18,7 +18,7 @@ def setUp(self):
         super().setUp()
         class P(param.Parameterized):
             e = param.ListSelector(default=[5],objects=[5,6,7])
-            f = param.ListSelector(default=10)
+            f = param.ListSelector(default=[10])
             h = param.ListSelector(default=None)
             g = param.ListSelector(default=None,objects=[7,8])
             i = param.ListSelector(default=[7],objects=[9],check_on_set=False)
@@ -173,16 +173,17 @@ class Q(param.Parameterized):
             r = param.ListSelector(default=[6])
 
     ##########################
-    # CEBALERT: not sure it makes sense for ListSelector to be set to
-    # a non-iterable value (except None). I.e. I think this first test
-    # should fail.
     def test_default_not_checked_to_be_iterable(self):
-        class Q(param.Parameterized):
-            r = param.ListSelector(default=6)
+        with pytest.raises(
+            ValueError,
+            match=re.escape("ListSelector parameter 'r' only takes list types, not 6."),
+        ):
+            class Q(param.Parameterized):
+                r = param.ListSelector(default=6)
 
     def test_set_checked_to_be_iterable(self):
         class Q(param.Parameterized):
-            r = param.ListSelector(default=6,check_on_set=False)
+            r = param.ListSelector(default=[6],check_on_set=False)
 
         with self.assertRaises(ValueError):
             Q.r = 6