diff --git a/image_prediction/estimator/preprocessor/preprocessor.py b/image_prediction/estimator/preprocessor/preprocessor.py
index 3959b51..6f39564 100644
--- a/image_prediction/estimator/preprocessor/preprocessor.py
+++ b/image_prediction/estimator/preprocessor/preprocessor.py
@@ -3,9 +3,8 @@ import abc
 
 class Preprocessor(abc.ABC):
 
-    @staticmethod
     @abc.abstractmethod
-    def preprocess(batch):
+    def preprocess(self, batch):
         raise NotImplementedError
 
     def __call__(self, batch):
diff --git a/image_prediction/image_extractor/extractor.py b/image_prediction/image_extractor/extractor.py
index 66bfc3b..fc318a8 100644
--- a/image_prediction/image_extractor/extractor.py
+++ b/image_prediction/image_extractor/extractor.py
@@ -6,10 +6,10 @@ ImageMetadataPair = namedtuple("ImageMetadataPair", ["image", "metadata"])
 
 
 class ImageExtractor(abc.ABC):
+
     @abc.abstractmethod
     def extract(self, obj) -> Iterable[ImageMetadataPair]:
-        """Extracts images from an object"""
-        pass
+        raise NotImplementedError
 
     def __call__(self, obj):
         return self.extract(obj)
diff --git a/image_prediction/label_mapper/mappers/numeric.py b/image_prediction/label_mapper/mappers/numeric.py
index 00e9f45..f08de75 100644
--- a/image_prediction/label_mapper/mappers/numeric.py
+++ b/image_prediction/label_mapper/mappers/numeric.py
@@ -9,7 +9,7 @@ class IndexMapper(LabelMapper):
         self.__labels = labels
 
     def __validate_index_label_format(self, index_label: int) -> None:
-        if not 0 <= index_label <= len(self.__labels):
+        if not 0 <= index_label < len(self.__labels):
             raise UnexpectedLabelFormat(
                 f"Received index label  '{index_label}' that has no associated string label."
             )
diff --git a/test/unit_tests/label_mapper_test.py b/test/unit_tests/label_mapper_test.py
index 83de74a..23da392 100644
--- a/test/unit_tests/label_mapper_test.py
+++ b/test/unit_tests/label_mapper_test.py
@@ -1,3 +1,6 @@
+import pytest
+
+from image_prediction.exceptions import UnexpectedLabelFormat
 from image_prediction.label_mapper.mappers.numeric import IndexMapper
 from image_prediction.label_mapper.mappers.probability import ProbabilityMapper
 
@@ -5,6 +8,8 @@ from image_prediction.label_mapper.mappers.probability import ProbabilityMapper
 def test_index_label_mapper(batch_of_expected_numeric_labels, batch_of_expected_string_labels, classes):
     mapper = IndexMapper(classes)
     assert list(mapper(batch_of_expected_numeric_labels)) == batch_of_expected_string_labels
+    with pytest.raises(UnexpectedLabelFormat):
+        list(mapper([len(classes)]))
 
 
 def test_array_label_mapper(
@@ -12,3 +17,5 @@ def test_array_label_mapper(
 ):
     mapper = ProbabilityMapper(classes)
     assert list(mapper(batch_of_expected_probability_arrays)) == batch_of_expected_label_to_probability_mappings
+    with pytest.raises(UnexpectedLabelFormat):
+        list(mapper([[0] * len(classes) + [1]]))