diff --git a/python/ffsim/variational/ucj_spin_balanced.py b/python/ffsim/variational/ucj_spin_balanced.py
index 0505c9135..c5641d99c 100644
--- a/python/ffsim/variational/ucj_spin_balanced.py
+++ b/python/ffsim/variational/ucj_spin_balanced.py
@@ -98,6 +98,12 @@ class UCJOpSpinBalanced:
     atol: InitVar[float] = 1e-8
 
     def __post_init__(self, validate: bool, rtol: float, atol: float):
+        if not np.iscomplexobj(self.orbital_rotations):
+            raise TypeError("Orbital rotations must have complex data type.")
+        if self.final_orbital_rotation is not None and not np.iscomplexobj(
+            self.final_orbital_rotation
+        ):
+            raise TypeError("Final orbital rotation must have complex data type.")
         if validate:
             if self.diag_coulomb_mats.ndim != 4 or self.diag_coulomb_mats.shape[1] != 2:
                 raise ValueError(
diff --git a/python/ffsim/variational/ucj_spin_unbalanced.py b/python/ffsim/variational/ucj_spin_unbalanced.py
index 13aa17ae5..3f7d6043c 100644
--- a/python/ffsim/variational/ucj_spin_unbalanced.py
+++ b/python/ffsim/variational/ucj_spin_unbalanced.py
@@ -100,6 +100,12 @@ class UCJOpSpinUnbalanced:
     atol: InitVar[float] = 1e-8
 
     def __post_init__(self, validate: bool, rtol: float, atol: float):
+        if not np.iscomplexobj(self.orbital_rotations):
+            raise TypeError("Orbital rotations must have complex data type.")
+        if self.final_orbital_rotation is not None and not np.iscomplexobj(
+            self.final_orbital_rotation
+        ):
+            raise TypeError("Final orbital rotation must have complex data type.")
         if validate:
             if self.diag_coulomb_mats.ndim != 4 or self.diag_coulomb_mats.shape[1] != 3:
                 raise ValueError(
diff --git a/python/ffsim/variational/ucj_spinless.py b/python/ffsim/variational/ucj_spinless.py
index 4e36fd9b9..6e9dfbde0 100644
--- a/python/ffsim/variational/ucj_spinless.py
+++ b/python/ffsim/variational/ucj_spinless.py
@@ -87,6 +87,12 @@ class UCJOpSpinless:
     atol: InitVar[float] = 1e-8
 
     def __post_init__(self, validate: bool, rtol: float, atol: float):
+        if not np.iscomplexobj(self.orbital_rotations):
+            raise TypeError("Orbital rotations must have complex data type.")
+        if self.final_orbital_rotation is not None and not np.iscomplexobj(
+            self.final_orbital_rotation
+        ):
+            raise TypeError("Final orbital rotation must have complex data type.")
         if validate:
             if self.diag_coulomb_mats.ndim != 3:
                 raise ValueError(
diff --git a/tests/python/variational/ucj_spin_balanced_test.py b/tests/python/variational/ucj_spin_balanced_test.py
index f92efcb71..f186d5e4e 100644
--- a/tests/python/variational/ucj_spin_balanced_test.py
+++ b/tests/python/variational/ucj_spin_balanced_test.py
@@ -214,7 +214,7 @@ def test_validate():
     norb = 4
     eye = np.eye(norb)
     diag_coulomb_mats = np.stack([np.stack([eye, eye]) for _ in range(n_reps)])
-    orbital_rotations = np.stack([eye for _ in range(n_reps)])
+    orbital_rotations = np.stack([eye.astype(complex) for _ in range(n_reps)])
 
     _ = ffsim.UCJOpSpinBalanced(
         diag_coulomb_mats=rng.standard_normal(10),
diff --git a/tests/python/variational/ucj_spin_unbalanced_test.py b/tests/python/variational/ucj_spin_unbalanced_test.py
index 4e08b530a..216d51279 100644
--- a/tests/python/variational/ucj_spin_unbalanced_test.py
+++ b/tests/python/variational/ucj_spin_unbalanced_test.py
@@ -249,7 +249,9 @@ def test_validate():
     norb = 4
     eye = np.eye(norb)
     diag_coulomb_mats = np.stack([np.stack([eye, eye, eye]) for _ in range(n_reps)])
-    orbital_rotations = np.stack([np.stack([eye, eye]) for _ in range(n_reps)])
+    orbital_rotations = np.stack(
+        [np.stack([eye, eye]).astype(complex) for _ in range(n_reps)]
+    )
 
     _ = ffsim.UCJOpSpinUnbalanced(
         diag_coulomb_mats=rng.standard_normal(10),
diff --git a/tests/python/variational/ucj_spinless_test.py b/tests/python/variational/ucj_spinless_test.py
index 2ec1ec785..538d365a4 100644
--- a/tests/python/variational/ucj_spinless_test.py
+++ b/tests/python/variational/ucj_spinless_test.py
@@ -219,7 +219,7 @@ def test_validate():
     norb = 4
     eye = np.eye(norb)
     diag_coulomb_mats = np.stack([eye for _ in range(n_reps)])
-    orbital_rotations = np.stack([eye for _ in range(n_reps)])
+    orbital_rotations = np.stack([eye.astype(complex) for _ in range(n_reps)])
 
     _ = ffsim.UCJOpSpinless(
         diag_coulomb_mats=rng.standard_normal(10),
@@ -241,13 +241,13 @@ def test_validate():
     with pytest.raises(ValueError, match="shape"):
         _ = ffsim.UCJOpSpinless(
             diag_coulomb_mats=diag_coulomb_mats,
-            orbital_rotations=rng.standard_normal(10),
+            orbital_rotations=rng.standard_normal(10).astype(complex),
         )
     with pytest.raises(ValueError, match="shape"):
         _ = ffsim.UCJOpSpinless(
             diag_coulomb_mats=diag_coulomb_mats,
             orbital_rotations=orbital_rotations,
-            final_orbital_rotation=rng.standard_normal(10),
+            final_orbital_rotation=rng.standard_normal(10).astype(complex),
         )
     with pytest.raises(ValueError, match="dimension"):
         _ = ffsim.UCJOpSpinless(
@@ -262,11 +262,11 @@ def test_validate():
     with pytest.raises(ValueError, match="unitary"):
         _ = ffsim.UCJOpSpinless(
             diag_coulomb_mats=diag_coulomb_mats,
-            orbital_rotations=rng.standard_normal((n_reps, norb, norb)),
+            orbital_rotations=rng.standard_normal((n_reps, norb, norb)).astype(complex),
         )
     with pytest.raises(ValueError, match="unitary"):
         _ = ffsim.UCJOpSpinless(
             diag_coulomb_mats=diag_coulomb_mats,
             orbital_rotations=orbital_rotations,
-            final_orbital_rotation=rng.standard_normal((norb, norb)),
+            final_orbital_rotation=rng.standard_normal((norb, norb)).astype(complex),
         )