diff --git a/src/datasets/features/features.py b/src/datasets/features/features.py
index 3e8301df08b..5e3df491d4b 100644
--- a/src/datasets/features/features.py
+++ b/src/datasets/features/features.py
@@ -854,6 +854,9 @@ class ClassLabel:
      * `names`: List of label strings.
      * `names_file`: File containing the list of labels.
 
+    Under the hood the labels are stored as integers.
+    You can use negative integers to represent unknown/missing labels.
+
     Args:
         num_classes (:obj:`int`, optional): Number of classes. All labels must be < `num_classes`.
         names (:obj:`list` of :obj:`str`, optional): String names for the integer classes.
@@ -910,7 +913,7 @@ def __post_init__(self, names_file):
     def __call__(self):
         return self.pa_type
 
-    def str2int(self, values: Union[str, Iterable]):
+    def str2int(self, values: Union[str, Iterable]) -> Union[int, Iterable]:
         """Conversion class name string => integer.
 
         Example:
@@ -934,7 +937,7 @@ def str2int(self, values: Union[str, Iterable]):
         output = [self._strval2int(value) for value in values]
         return output if return_list else output[0]
 
-    def _strval2int(self, value: str):
+    def _strval2int(self, value: str) -> int:
         failed_parse = False
         value = str(value)
         # first attempt - raw string value
@@ -955,9 +958,11 @@ def _strval2int(self, value: str):
             raise ValueError(f"Invalid string class label {value}")
         return int_value
 
-    def int2str(self, values: Union[int, Iterable]):
+    def int2str(self, values: Union[int, Iterable]) -> Union[str, Iterable]:
         """Conversion integer => class name string.
 
+        Regarding unknown/missing labels: passing negative integers raises ValueError.
+
         Example:
 
         ```py
@@ -1014,8 +1019,6 @@ def cast_storage(self, storage: Union[pa.StringArray, pa.IntegerArray]) -> pa.In
         """
         if isinstance(storage, pa.IntegerArray):
             min_max = pc.min_max(storage).as_py()
-            if min_max["min"] < -1:
-                raise ValueError(f"Class label {min_max['min']} less than -1")
             if min_max["max"] >= self.num_classes:
                 raise ValueError(
                     f"Class label {min_max['max']} greater than configured num_classes {self.num_classes}"
diff --git a/tests/features/test_features.py b/tests/features/test_features.py
index a800ba3419f..1868e067aa8 100644
--- a/tests/features/test_features.py
+++ b/tests/features/test_features.py
@@ -288,6 +288,8 @@ def test_classlabel_str2int():
         classlabel.str2int("__bad_label_name__")
     with pytest.raises(ValueError):
         classlabel.str2int(1)
+    with pytest.raises(ValueError):
+        classlabel.str2int(None)
 
 
 def test_classlabel_int2str():
@@ -297,6 +299,45 @@ def test_classlabel_int2str():
         assert classlabel.int2str(i) == names[i]
     with pytest.raises(ValueError):
         classlabel.int2str(len(names))
+    with pytest.raises(ValueError):
+        classlabel.int2str(-1)
+    with pytest.raises(ValueError):
+        classlabel.int2str(None)
+
+
+def test_classlabel_cast_storage():
+    names = ["negative", "positive"]
+    classlabel = ClassLabel(names=names)
+    # from integers
+    arr = pa.array([0, 1, -1, -100], type=pa.int64())
+    result = classlabel.cast_storage(arr)
+    assert result.type == pa.int64()
+    assert result.to_pylist() == [0, 1, -1, -100]
+    arr = pa.array([0, 1, -1, -100], type=pa.int32())
+    result = classlabel.cast_storage(arr)
+    assert result.type == pa.int64()
+    assert result.to_pylist() == [0, 1, -1, -100]
+    arr = pa.array([3])
+    with pytest.raises(ValueError):
+        classlabel.cast_storage(arr)
+    # from strings
+    arr = pa.array(["negative", "positive"])
+    result = classlabel.cast_storage(arr)
+    assert result.type == pa.int64()
+    assert result.to_pylist() == [0, 1]
+    arr = pa.array(["__label_that_doesnt_exist__"])
+    with pytest.raises(ValueError):
+        classlabel.cast_storage(arr)
+    # from nulls
+    arr = pa.array([None])
+    result = classlabel.cast_storage(arr)
+    assert result.type == pa.int64()
+    assert result.to_pylist() == [None]
+    # from empty
+    arr = pa.array([])
+    result = classlabel.cast_storage(arr)
+    assert result.type == pa.int64()
+    assert result.to_pylist() == []
 
 
 @pytest.mark.parametrize("class_label_arg", ["names", "names_file"])