diff --git a/src/roboticstoolbox/ets/ETS.py b/src/roboticstoolbox/ets/ETS.py
index 2be215940..c4ad72701 100644
--- a/src/roboticstoolbox/ets/ETS.py
+++ b/src/roboticstoolbox/ets/ETS.py
@@ -175,7 +175,12 @@ def fixjoint(self, j: int, value: float, unit: str = "rad") -> ETS:
         :returns: ETS with one fewer joint
         :raises ValueError: for an invalid joint position or angular unit
         """
-        if not isinstance(j, (int, np.integer)) or not 0 <= j < self.n:
+        # bool subclasses int, but must not select joint 0 or 1.
+        if (
+            isinstance(j, bool)
+            or not isinstance(j, (int, np.integer))
+            or not 0 <= j < self.n
+        ):
             raise ValueError(f"joint index {j} out of range for {self.n} joints")
         if unit not in ("rad", "deg"):
             raise ValueError("unit must be 'rad' or 'deg'")
diff --git a/src/roboticstoolbox/ets/ETS2.py b/src/roboticstoolbox/ets/ETS2.py
index 8261a0abe..79ffebb1c 100644
--- a/src/roboticstoolbox/ets/ETS2.py
+++ b/src/roboticstoolbox/ets/ETS2.py
@@ -147,7 +147,12 @@ def fixjoint(self, j: int, value: float, unit: str = "rad") -> ETS2:
         :returns: ETS2 with one fewer joint
         :raises ValueError: for an invalid joint position or angular unit
         """
-        if not isinstance(j, (int, np.integer)) or not 0 <= j < self.n:
+        # bool subclasses int, but must not select joint 0 or 1.
+        if (
+            isinstance(j, bool)
+            or not isinstance(j, (int, np.integer))
+            or not 0 <= j < self.n
+        ):
             raise ValueError(f"joint index {j} out of range for {self.n} joints")
         if unit not in ("rad", "deg"):
             raise ValueError("unit must be 'rad' or 'deg'")
diff --git a/tests/test_ETS.py b/tests/test_ETS.py
index bb02c8440..72b3746c3 100644
--- a/tests/test_ETS.py
+++ b/tests/test_ETS.py
@@ -231,6 +231,23 @@ def test_fixjoint_rotation_in_degrees(self):
             ets.fixjoint(0, 90, unit="grad")
         nt.assert_allclose(ets.eval(q), expected)
 
+    def test_fixjoint_boolean_and_integer_indices(self):
+        ets = Rz() * tx()
+        q = np.array([0.2, 0.3])
+        expected = ets.eval(q)
+
+        # Both 0 and 1 are valid positions, so rejection must be based on type.
+        for index in (False, True, np.bool_(False), np.bool_(True)):
+            with self.subTest(index=index), self.assertRaises(ValueError):
+                ets.fixjoint(index, 0.2)
+
+        for index in (0, 1, np.int64(0), np.int64(1)):
+            with self.subTest(index=index):
+                fixed = ets.fixjoint(index, q[index])
+                nt.assert_allclose(fixed.eval(np.delete(q, index)), expected)
+        self.assertEqual(ets.n, 2)
+        nt.assert_allclose(ets.eval(q), expected)
+
     def test_fixjoint_single_joint_and_bad_indices(self):
         ets = rtb.ETS(Rz())
         fixed = ets.fixjoint(0, 0.3)
diff --git a/tests/test_ETS2.py b/tests/test_ETS2.py
index fb683eac1..4d48b230c 100644
--- a/tests/test_ETS2.py
+++ b/tests/test_ETS2.py
@@ -230,6 +230,23 @@ def test_fixjoint_rotation_in_degrees(self):
             ets.fixjoint(0, 90, unit="grad")
         nt.assert_allclose(ets.eval(q), expected)
 
+    def test_fixjoint_boolean_and_integer_indices(self):
+        ets = R() * tx()
+        q = np.array([0.2, 0.3])
+        expected = ets.eval(q)
+
+        # Both 0 and 1 are valid positions, so rejection must be based on type.
+        for index in (False, True, np.bool_(False), np.bool_(True)):
+            with self.subTest(index=index), self.assertRaises(ValueError):
+                ets.fixjoint(index, 0.2)
+
+        for index in (0, 1, np.int64(0), np.int64(1)):
+            with self.subTest(index=index):
+                fixed = ets.fixjoint(index, q[index])
+                nt.assert_allclose(fixed.eval(np.delete(q, index)), expected)
+        self.assertEqual(ets.n, 2)
+        nt.assert_allclose(ets.eval(q), expected)
+
     def test_fixjoint_single_joint_and_bad_indices(self):
         ets = rtb.ETS2(R())
         fixed = ets.fixjoint(0, 0.3)