diff --git a/docs/examples/plot_cast_transformer.py b/docs/examples/plot_cast_transformer.py
index 34efc74f8..82cfef35e 100644
--- a/docs/examples/plot_cast_transformer.py
+++ b/docs/examples/plot_cast_transformer.py
@@ -30,7 +30,6 @@
 """
 import onnxruntime
 import onnx
-import numpy
 import os
 import math
 import numpy as np
diff --git a/docs/examples/plot_tfidfvectorizer.py b/docs/examples/plot_tfidfvectorizer.py
index 96321bfbf..290eb5e36 100644
--- a/docs/examples/plot_tfidfvectorizer.py
+++ b/docs/examples/plot_tfidfvectorizer.py
@@ -24,7 +24,6 @@
 import matplotlib.pyplot as plt
 import os
 from onnx.tools.net_drawer import GetPydotGraph, GetOpNodeProducer
-import numpy
 import onnxruntime as rt
 from skl2onnx.common.data_types import StringTensorType
 from skl2onnx import convert_sklearn
diff --git a/skl2onnx/operator_converters/decision_tree.py b/skl2onnx/operator_converters/decision_tree.py
index e92fe285c..e0a6326f8 100644
--- a/skl2onnx/operator_converters/decision_tree.py
+++ b/skl2onnx/operator_converters/decision_tree.py
@@ -3,6 +3,7 @@
 
 import numbers
 import numpy as np
+from onnx.numpy_helper import from_array
 from ..common._apply_operation import (
     apply_cast,
     apply_concat,
@@ -124,7 +125,7 @@ def predict(
             [indices_name, dummy_proba_name],
             op_domain=op_domain,
             op_version=op_version,
-            **attrs
+            **attrs,
         )
     else:
         zero_name = scope.get_unique_variable_name("zero")
@@ -243,7 +244,7 @@ def _append_decision_output(
         dpath,
         op_domain=op_domain,
         op_version=op_version,
-        **attrs
+        **attrs,
     )
 
     if n_out is None:
@@ -306,6 +307,53 @@ def convert_sklearn_decision_tree_classifier(
         dtype = np.float32
     op = operator.raw_operator
     options = scope.get_options(op, dict(decision_path=False, decision_leaf=False))
+    if np.asarray(op.classes_).size == 1:
+        # The model was trained with one label.
+        # There is no need to build a tree.
+        if op.n_outputs_ != 1:
+            raise RuntimeError(
+                f"One training class and multiple outputs is not "
+                f"supported yet for class {op.__class__.__name__!r}."
+            )
+        if options["decision_path"] or options["decision_leaf"]:
+            raise RuntimeError(
+                f"One training class, option 'decision_path' "
+                f"or 'decision_leaf' are not supported for "
+                f"class {op.__class__.__name__!r}."
+            )
+
+        zero = scope.get_unique_variable_name("zero")
+        one = scope.get_unique_variable_name("one")
+        new_shape = scope.get_unique_variable_name("new_shape")
+        container.add_initializer(zero, onnx_proto.TensorProto.INT64, [1], [0])
+        container.add_initializer(one, onnx_proto.TensorProto.INT64, [1], [1])
+        container.add_initializer(new_shape, onnx_proto.TensorProto.INT64, [2], [-1, 1])
+        shape = scope.get_unique_variable_name("shape")
+        container.add_node("Shape", [operator.inputs[0].full_name], [shape])
+        shape_sliced = scope.get_unique_variable_name("shape_sliced")
+        container.add_node("Slice", [shape, zero, one, zero], [shape_sliced])
+
+        # labels
+        container.add_node(
+            "ConstantOfShape",
+            [shape_sliced],
+            [operator.outputs[0].full_name],
+            value=from_array(np.array([op.classes_[0]], dtype=np.int64)),
+        )
+
+        # probabilities
+        probas = scope.get_unique_variable_name("probas")
+        container.add_node(
+            "ConstantOfShape",
+            [shape_sliced],
+            [probas],
+            value=from_array(np.array([1], dtype=dtype)),
+        )
+        container.add_node(
+            "Reshape", [probas, new_shape], [operator.outputs[1].full_name]
+        )
+        return
+
     if op.n_outputs_ == 1:
         attrs = get_default_tree_classifier_attribute_pairs()
         attrs["name"] = scope.get_unique_operator_name(op_type)
@@ -355,7 +403,7 @@ def convert_sklearn_decision_tree_classifier(
             [operator.outputs[0].full_name, operator.outputs[1].full_name],
             op_domain=op_domain,
             op_version=op_version,
-            **attrs
+            **attrs,
         )
 
         n_out = 2
@@ -510,7 +558,7 @@ def convert_sklearn_decision_tree_regressor(
         operator.outputs[0].full_name,
         op_domain=op_domain,
         op_version=op_version,
-        **attrs
+        **attrs,
     )
 
     options = scope.get_options(op, dict(decision_path=False, decision_leaf=False))
diff --git a/tests/test_sklearn_classifiers_extreme.py b/tests/test_sklearn_classifiers_extreme.py
new file mode 100644
index 000000000..d26ca31a5
--- /dev/null
+++ b/tests/test_sklearn_classifiers_extreme.py
@@ -0,0 +1,45 @@
+# SPDX-License-Identifier: Apache-2.0
+
+import unittest
+import numpy as np
+
+try:
+    from onnx.reference import ReferenceEvaluator
+except ImportError:
+    ReferenceEvaluator = None
+from sklearn.tree import DecisionTreeClassifier
+from onnxruntime import InferenceSession
+from skl2onnx import to_onnx
+from test_utils import TARGET_OPSET
+
+
+class TestSklearnClassifiersExtreme(unittest.TestCase):
+    def test_one_training_class(self):
+        x = np.eye(4, dtype=np.float32)
+        y = np.array([5, 5, 5, 5], dtype=np.int64)
+
+        cl = DecisionTreeClassifier()
+        cl = cl.fit(x, y)
+
+        expected = [cl.predict(x), cl.predict_proba(x)]
+        onx = to_onnx(cl, x, target_opset=TARGET_OPSET, options={"zipmap": False})
+
+        for cls in [
+            (lambda onx: ReferenceEvaluator(onx, verbose=0))
+            if ReferenceEvaluator is not None
+            else None,
+            lambda onx: InferenceSession(
+                onx.SerializeToString(), providers=["CPUExecutionProvider"]
+            ),
+        ]:
+            if cls is None:
+                continue
+            sess = cls(onx)
+            res = sess.run(None, {"X": x})
+            self.assertEqual(len(res), len(expected))
+            for e, g in zip(expected, res):
+                self.assertEqual(e.tolist(), g.tolist())
+
+
+if __name__ == "__main__":
+    unittest.main(verbosity=2)