diff --git a/dpdata/vasp/poscar.py b/dpdata/vasp/poscar.py
index 075d8f2be..30073e2b3 100644
--- a/dpdata/vasp/poscar.py
+++ b/dpdata/vasp/poscar.py
@@ -121,12 +121,11 @@ def from_system_data(system, f_idx=0, skip_zeros=True):
         line = f"{ii_posi[0]:15.10f} {ii_posi[1]:15.10f} {ii_posi[2]:15.10f}"
         if move is not None and len(move) > 0:
             move_flags = move[idx]
-            if isinstance(move_flags, list) and len(move_flags) == 3:
-                line += " " + " ".join(["T" if flag else "F" for flag in move_flags])
-            else:
+            if not isinstance(move_flags, list) or len(move_flags) != 3:
                 raise RuntimeError(
                     f"Invalid move flags: {move_flags}, should be a list of 3 bools"
                 )
+            line += " " + " ".join("T" if flag else "F" for flag in move_flags)
 
         posi_list.append(line)
 
diff --git a/tests/test_vasp_poscar_to_system.py b/tests/test_vasp_poscar_to_system.py
index 23b1e982b..16cd64b83 100644
--- a/tests/test_vasp_poscar_to_system.py
+++ b/tests/test_vasp_poscar_to_system.py
@@ -19,7 +19,7 @@ def test_move_flags(self):
         self.assertTrue(np.array_equal(self.system["move"], expected))
 
 
-class TestPOSCARCart(unittest.TestCase):
+class TestPOSCARMoveFlags(unittest.TestCase):
     def test_move_flags_error1(self):
         with self.assertRaisesRegex(RuntimeError, "Invalid move flags.*?"):
             dpdata.System().from_vasp_poscar(os.path.join("poscars", "POSCAR.oh.err1"))