diff --git a/molpipeline/estimators/chemprop/models.py b/molpipeline/estimators/chemprop/models.py
index af4c577c..cd5c21a6 100644
--- a/molpipeline/estimators/chemprop/models.py
+++ b/molpipeline/estimators/chemprop/models.py
@@ -193,7 +193,7 @@ def fit(
             The fitted model.
         """
         if self._is_classifier():
-            self._classes_ = np.unique(y)
+            self._classes_ = np.unique(list(y))
         return super().fit(X, y)
 
     def predict(