diff --git a/sktree/tree/_honest_tree.py b/sktree/tree/_honest_tree.py
index 14f90cd99..cd91254f7 100644
--- a/sktree/tree/_honest_tree.py
+++ b/sktree/tree/_honest_tree.py
@@ -404,6 +404,7 @@ def partial_fit(self, X, y, sample_weight=None, check_input=True, classes=None):
                 classes=classes,
             )
             return self
+
         rng = np.random.default_rng(self.random_state)
 
         if sample_weight is None:
@@ -423,7 +424,7 @@ def partial_fit(self, X, y, sample_weight=None, check_input=True, classes=None):
         _sample_weight[self.honest_indices_] = 0
 
         if classes is None:
-            classes = np.unique(y)
+            classes = np.unique(y).tolist()
 
         self.estimator_.partial_fit(
             X,
@@ -560,7 +561,7 @@ def _fit(
             self.estimator_ = deepcopy(self.tree_estimator)
 
         if classes is None:
-            classes = np.unique(y)
+            classes = np.unique(y).tolist()
 
         # Learn structure on subsample
         self.estimator_._fit(