diff --git a/postprocessor.py b/postprocessor.py
index 627359c..c788f50 100644
--- a/postprocessor.py
+++ b/postprocessor.py
@@ -1298,7 +1298,7 @@ class PostProcessor(HDF5Container):
 
     def _write_cells(self):
         """Ensure cells are written in the hdf5 file"""
-        if os.path.exists(self.cells_filename) and not self.cells_loaded:
+        if not os.path.exists(self.cells_filename) and not self.cells_loaded:
             self.load_cells()
             self.unload_cells()
         return np.array([self.cells_filename])