diff --git a/src/skprometheus/preprocessing.py b/src/skprometheus/preprocessing.py
index 1dd7711..034d338 100644
--- a/src/skprometheus/preprocessing.py
+++ b/src/skprometheus/preprocessing.py
@@ -31,7 +31,7 @@ def transform(self, X):
 
         for idx, row in enumerate(categories.T):
             for category in row:
-                if not category:
+                if category is None:
                     category = "missing"
                 MetricRegistry.model_categorical(feature=str(features[idx]), category=str(category)).inc()
 
diff --git a/tests/test_preprocessing.py b/tests/test_preprocessing.py
index 864b5d7..f1169b6 100644
--- a/tests/test_preprocessing.py
+++ b/tests/test_preprocessing.py
@@ -37,6 +37,7 @@ def test_OneHotEncoder():
 
     assert REGISTRY.get_sample_value('skprom_model_categorical_total', {'feature': '2', 'category': '4'}) == 2
     assert REGISTRY.get_sample_value('skprom_model_categorical_total', {'feature': '3', 'category': '9'}) == 1
+    assert REGISTRY.get_sample_value('skprom_model_categorical_total', {'feature': '3', 'category': '0'}) == 1
 
 
 def test_OneHotEncoder_pandas():