diff --git a/dpti/lib/dump.py b/dpti/lib/dump.py
index 68c9940..55d4759 100644
--- a/dpti/lib/dump.py
+++ b/dpti/lib/dump.py
@@ -82,14 +82,18 @@ def get_posi(lines):
 
 
 def get_dumpbox(lines):
+    """Return LAMMPS dump bounds and optional triclinic tilt factors."""
     blk, h = _get_block(lines, "BOX BOUNDS")
     bounds = np.zeros([3, 2])
     tilt = np.zeros([3])
     for dd in range(3):
         info = [float(jj) for jj in blk[dd].split()]
+        if len(info) < 2:
+            raise ValueError("each BOX BOUNDS line must contain lower and upper bounds")
         bounds[dd][0] = info[0]
         bounds[dd][1] = info[1]
-        tilt[dd] = info[2]
+        if len(info) >= 3:
+            tilt[dd] = info[2]
     return bounds, tilt
 
 
diff --git a/tests/test_dump.py b/tests/test_dump.py
new file mode 100644
index 0000000..e3c53d7
--- /dev/null
+++ b/tests/test_dump.py
@@ -0,0 +1,26 @@
+import unittest
+
+import numpy as np
+
+from dpti.lib.dump import get_dumpbox
+
+
+class TestDumpBox(unittest.TestCase):
+    def test_orthogonal_box_defaults_to_zero_tilt(self):
+        """Standard two-column BOX BOUNDS lines describe an orthogonal box."""
+        lines = [
+            "ITEM: BOX BOUNDS pp pp pp",
+            "0 10",
+            "-1 9",
+            "2 12",
+            "ITEM: ATOMS id type x y z",
+        ]
+
+        bounds, tilt = get_dumpbox(lines)
+
+        np.testing.assert_allclose(bounds, [[0, 10], [-1, 9], [2, 12]])
+        np.testing.assert_allclose(tilt, [0, 0, 0])
+
+
+if __name__ == "__main__":
+    unittest.main()