diff --git a/dpdata/vasp/outcar.py b/dpdata/vasp/outcar.py
index e8280d056..0eddac91a 100644
--- a/dpdata/vasp/outcar.py
+++ b/dpdata/vasp/outcar.py
@@ -142,7 +142,12 @@ def analyze_block(lines, ntot, nelm, ml=False):
                 is_converge = False
         elif energy_token[ml_index] in ii:
             energy = float(ii.split()[energy_index[ml_index]])
-            assert (force is not None) and len(coord) > 0 and len(cell) > 0
+            if len(force) == 0:
+                raise ValueError("cannot find forces in OUTCAR block")
+            if len(coord) == 0:
+                raise ValueError("cannot find coordinates in OUTCAR block")
+            if len(cell) == 0:
+                raise ValueError("cannot find cell in OUTCAR block")
             return coord, cell, energy, force, virial, is_converge
         elif cell_token[ml_index] in ii:
             for dd in range(3):