diff --git a/tat/tensor.py b/tat/tensor.py
index 69af96c36..c5caa7eb3 100644
--- a/tat/tensor.py
+++ b/tat/tensor.py
@@ -583,6 +583,8 @@ def __setitem__(self: Tensor, position: tuple[int, ...] | tuple[slice, ...] | di
         indices = self._prepare_position(position)
         if self.mask[indices]:
             self.data[indices] = value
+        else:
+            raise IndexError("The indices specified are masked, so it is invalid to set item here.")
 
     def clear_symmetry(self: Tensor) -> Tensor:
         """
diff --git a/tests/test_create_tensor.py b/tests/test_create_tensor.py
index 001384d78..f784539c8 100644
--- a/tests/test_create_tensor.py
+++ b/tests/test_create_tensor.py
@@ -44,7 +44,11 @@ def test_tensor_get_set_item() -> None:
     assert a[0, 0] == 1
     a["i":2, "j":3] = 2  # type: ignore[misc]
     assert a[{"i": 2, "j": 3}] == 2
-    a[2, 0] = 3
+    try:
+        a[2, 0] = 3
+        assert False
+    except IndexError:
+        pass
     assert a["i":2, "j":0] == 0  # type: ignore[misc]
 
     b = tat.Tensor(
@@ -61,7 +65,11 @@ def test_tensor_get_set_item() -> None:
     assert b[0, 0] == 1
     b["i":2, "j":3] = 2  # type: ignore[misc]
     assert b[{"i": 2, "j": 3}] == 2
-    b[2, 0] = 3
+    try:
+        b[2, 0] = 3
+        assert False
+    except IndexError:
+        pass
     assert b["i":2, "j":0] == 0  # type: ignore[misc]