diff --git a/src/pyrecest/filters/se2_ukf.py b/src/pyrecest/filters/se2_ukf.py
index 5dc7df5b1..41939b5df 100644
--- a/src/pyrecest/filters/se2_ukf.py
+++ b/src/pyrecest/filters/se2_ukf.py
@@ -47,6 +47,15 @@ def _to_python_bool(value):
     return bool(value)
 
 
+def _is_complex_array(value):
+    """Return whether a NumPy/JAX array or PyTorch tensor has complex dtype."""
+    dtype = getattr(value, "dtype", None)
+    if getattr(dtype, "kind", None) == "c":
+        return True
+    is_complex = getattr(value, "is_complex", None)
+    return bool(is_complex()) if callable(is_complex) else False
+
+
 def _normalize_rotation_columns(rotation_samples, fallback_rotation):
     """Normalize 2-D rotation columns, replacing undefined zero directions."""
     rotation_samples = asarray(rotation_samples)
@@ -72,6 +81,10 @@ def _validate_se2_gaussian(distribution, role):
 
     mu = asarray(distribution.mu)
     covariance = asarray(distribution.C)
+    if _is_complex_array(mu):
+        raise ValueError(f"{role} mean must be real-valued.")
+    if _is_complex_array(covariance):
+        raise ValueError(f"{role} covariance must be real-valued.")
     if mu.shape != (4,):
         raise ValueError(f"{role} mean must be a 4-D vector.")
     if covariance.shape != (4, 4):
@@ -95,6 +108,8 @@ def _validate_se2_gaussian(distribution, role):
 
 def _validate_se2_measurement(z):
     measurement = asarray(z)
+    if _is_complex_array(measurement):
+        raise ValueError("measurement z must be real-valued.")
     if measurement.shape != (4,):
         raise ValueError("measurement z must be a 4-D vector.")
     if not _to_python_bool(backend_all(isfinite(measurement))):
diff --git a/tests/filters/test_se2_ukf_real_inputs.py b/tests/filters/test_se2_ukf_real_inputs.py
new file mode 100644
index 000000000..ea2e25c61
--- /dev/null
+++ b/tests/filters/test_se2_ukf_real_inputs.py
@@ -0,0 +1,57 @@
+"""Regression tests for real-valued SE(2) UKF inputs."""
+
+import unittest
+
+import numpy.testing as npt
+
+# pylint: disable=no-name-in-module,no-member
+import pyrecest.backend
+from pyrecest.backend import array, eye, to_numpy
+from pyrecest.distributions import GaussianDistribution
+from pyrecest.filters.se2_ukf import SE2UKF
+
+
+@unittest.skipIf(
+    pyrecest.backend.__backend_name__ == "jax",
+    reason="SE2UKF update is not supported on JAX",
+)
+class TestSE2UKFRealInputs(unittest.TestCase):
+    @staticmethod
+    def _noise_distribution():
+        return GaussianDistribution(
+            array([1.0, 0.0, 0.0, 0.0]),
+            0.1 * eye(4),
+        )
+
+    def test_update_rejects_complex_measurement_without_mutating_state(self):
+        current_filter = SE2UKF()
+        original_mean = to_numpy(current_filter.filter_state.mu).copy()
+        original_covariance = to_numpy(current_filter.filter_state.C).copy()
+
+        with self.assertRaisesRegex(ValueError, "real-valued"):
+            current_filter.update_identity(
+                self._noise_distribution(),
+                array([1.0 + 0.0j, 0.0, 1.0j, 0.0]),
+            )
+
+        npt.assert_allclose(
+            to_numpy(current_filter.filter_state.mu), original_mean
+        )
+        npt.assert_allclose(
+            to_numpy(current_filter.filter_state.C), original_covariance
+        )
+
+    def test_filter_state_rejects_complex_mean_direction(self):
+        current_filter = SE2UKF()
+        invalid_state = GaussianDistribution(
+            array([1.0, 0.0, 0.0, 0.0]),
+            eye(4),
+        )
+        invalid_state.mu = array([1.0 + 0.0j, 0.0, 1.0j, 0.0])
+
+        with self.assertRaisesRegex(ValueError, "real-valued"):
+            current_filter.filter_state = invalid_state
+
+
+if __name__ == "__main__":
+    unittest.main()