diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8f0ffb99f..fd4f9c9cf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -52,6 +52,7 @@ Attention: The newest changes should be on top -->
 
 ### Fixed
 
+- BUG: correct encoding for trapezoidal sweep length and angle. [#861](https://github.com/RocketPy-Team/RocketPy/pull/861)
 - BUG: Fix no time initialization when passing initial_solution as array to Flight object [#844](https://github.com/RocketPy-Team/RocketPy/pull/844)
 
 
diff --git a/rocketpy/plots/monte_carlo_plots.py b/rocketpy/plots/monte_carlo_plots.py
index d602b3a46..22d320b6b 100644
--- a/rocketpy/plots/monte_carlo_plots.py
+++ b/rocketpy/plots/monte_carlo_plots.py
@@ -1,6 +1,6 @@
 import matplotlib.pyplot as plt
-from matplotlib.transforms import offset_copy
 import numpy as np
+from matplotlib.transforms import offset_copy
 
 from ..tools import generate_monte_carlo_ellipses, import_optional_dependency
 
diff --git a/rocketpy/rocket/aero_surface/fins/trapezoidal_fins.py b/rocketpy/rocket/aero_surface/fins/trapezoidal_fins.py
index 58b68799f..c6b4ea633 100644
--- a/rocketpy/rocket/aero_surface/fins/trapezoidal_fins.py
+++ b/rocketpy/rocket/aero_surface/fins/trapezoidal_fins.py
@@ -176,9 +176,6 @@ def __init__(
             sweep_length = np.tan(sweep_angle * np.pi / 180) * span
         elif sweep_length is None:
             sweep_length = root_chord - tip_chord
-        else:
-            # Sweep length is given
-            pass
 
         self._tip_chord = tip_chord
         self._sweep_length = sweep_length
@@ -351,12 +348,12 @@ def all_info(self):
     def to_dict(self, **kwargs):
         data = super().to_dict(**kwargs)
         data["tip_chord"] = self.tip_chord
+        data["sweep_length"] = self.sweep_length
+        data["sweep_angle"] = self.sweep_angle
 
         if kwargs.get("include_outputs", False):
             data.update(
                 {
-                    "sweep_length": self.sweep_length,
-                    "sweep_angle": self.sweep_angle,
                     "shape_vec": self.shape_vec,
                     "Af": self.Af,
                     "AR": self.AR,
@@ -382,4 +379,5 @@ def from_dict(cls, data):
             cant_angle=data["cant_angle"],
             airfoil=data["airfoil"],
             name=data["name"],
+            sweep_length=data.get("sweep_length"),
         )
diff --git a/tests/fixtures/surfaces/surface_fixtures.py b/tests/fixtures/surfaces/surface_fixtures.py
index bf6e384c4..c48627478 100644
--- a/tests/fixtures/surfaces/surface_fixtures.py
+++ b/tests/fixtures/surfaces/surface_fixtures.py
@@ -69,6 +69,50 @@ def calisto_trapezoidal_fins():
     )
 
 
+@pytest.fixture
+def calisto_trapezoidal_fins_custom_sweep_length():
+    """The trapezoidal fins of the Calisto rocket with
+    a custom sweep length.
+
+    Returns
+    -------
+    rocketpy.TrapezoidalFins
+        The trapezoidal fins of the Calisto rocket.
+    """
+    return TrapezoidalFins(
+        n=4,
+        span=0.100,
+        root_chord=0.120,
+        tip_chord=0.040,
+        rocket_radius=0.0635,
+        name="calisto_trapezoidal_fins_custom_sweep_length",
+        cant_angle=0,
+        sweep_length=0.1,
+    )
+
+
+@pytest.fixture
+def calisto_trapezoidal_fins_custom_sweep_angle():
+    """The trapezoidal fins of the Calisto rocket with
+    a custom sweep angle.
+
+    Returns
+    -------
+    rocketpy.TrapezoidalFins
+        The trapezoidal fins of the Calisto rocket.
+    """
+    return TrapezoidalFins(
+        n=4,
+        span=0.100,
+        root_chord=0.120,
+        tip_chord=0.040,
+        rocket_radius=0.0635,
+        name="calisto_trapezoidal_fins_custom_sweep_angle",
+        cant_angle=0,
+        sweep_angle=30,
+    )
+
+
 @pytest.fixture
 def calisto_free_form_fins():
     """The free form fins of the Calisto rocket.
diff --git a/tests/integration/test_encoding.py b/tests/integration/test_encoding.py
index 90e454af1..38d5aef41 100644
--- a/tests/integration/test_encoding.py
+++ b/tests/integration/test_encoding.py
@@ -242,6 +242,41 @@ def test_rocket_encoder(rocket_name, request):
     )
 
 
+@pytest.mark.parametrize(
+    "fin_name",
+    [
+        "calisto_trapezoidal_fins",
+        "calisto_trapezoidal_fins_custom_sweep_length",
+        "calisto_trapezoidal_fins_custom_sweep_angle",
+    ],
+)
+def test_trapezoidal_fins_encoder(fin_name, request):
+    """Test encoding a ``rocketpy.TrapezoidalFins``.
+
+    Parameters
+    ----------
+    fin_name : str
+        Name of the fin fixture to encode.
+    request : pytest.FixtureRequest
+        Pytest request object.
+    """
+    fin_to_encode = request.getfixturevalue(fin_name)
+
+    json_encoded = json.dumps(fin_to_encode, cls=RocketPyEncoder)
+
+    fin_loaded = json.loads(json_encoded, cls=RocketPyDecoder)
+
+    assert isinstance(fin_loaded, type(fin_to_encode))
+    assert fin_to_encode.n == fin_loaded.n
+    assert np.isclose(fin_to_encode.span, fin_loaded.span)
+    assert np.isclose(fin_to_encode.root_chord, fin_loaded.root_chord)
+    assert np.isclose(fin_to_encode.tip_chord, fin_loaded.tip_chord)
+    assert np.isclose(fin_to_encode.rocket_radius, fin_loaded.rocket_radius)
+    assert np.isclose(fin_to_encode.sweep_length, fin_loaded.sweep_length)
+    if fin_to_encode._sweep_angle is not None and fin_loaded._sweep_angle is not None:
+        assert np.isclose(fin_to_encode.sweep_angle, fin_loaded.sweep_angle)
+
+
 @pytest.mark.parametrize("rocket_name", ["calisto_robust", "calisto_hybrid_modded"])
 def test_encoder_discretize(rocket_name, request):
     """Test encoding the total mass of ``rocketpy.Rocket`` with