diff --git a/src/pyiron_lammps/compatibility/calculate.py b/src/pyiron_lammps/compatibility/calculate.py
index d9853c0..c77d562 100644
--- a/src/pyiron_lammps/compatibility/calculate.py
+++ b/src/pyiron_lammps/compatibility/calculate.py
@@ -403,9 +403,11 @@ def _pressure_to_lammps(pressure, rotation_matrix, units="metal"):
 
     # If necessary, rotate the pressure tensor to the Lammps coordinate frame.
     # Isotropic, hydrostatic pressures are rotation invariant.
-    if not np.isclose(
-        np.matrix.trace(rotation_matrix), 3
-    ) and not _is_isotropic_hydrostatic(pressure):
+    if (
+        rotation_matrix is not None
+        and not np.isclose(np.matrix.trace(rotation_matrix), 3)
+        and not _is_isotropic_hydrostatic(pressure)
+    ):
         if any(p is None for p in pressure):
             raise ValueError(
                 "Cells which are not orthorhombic or an upper-triangular cell are incompatible with Lammps "