diff --git a/spot/spotter.py b/spot/spotter.py
index 774c8dc..f9ea35d 100644
--- a/spot/spotter.py
+++ b/spot/spotter.py
@@ -168,10 +168,11 @@ def _check_propagation_options(self):
         # Sanity check:
         msg = ''
         for bfc_mod in self.propagation_options['N_rigid_mod_diff_bfc']:
-            if bfc_mod not in self.propagation_options['N_rigid_modalities']:
-                msg += '\n\n === The modality selected for the differential bias field is not present in ' \
-                       'the stack. Turn N_rigid_mod_diff_bfc to empty list, or add its values to ' \
-                       'N_rigid_modalities == \n'
+            if len(self.propagation_options['N_rigid_modalities']) > 0:
+                if bfc_mod not in self.propagation_options['N_rigid_modalities']:
+                    msg += '\n\n === The modality selected for the differential bias field is not present in ' \
+                           'the stack. Turn N_rigid_mod_diff_bfc to empty list, or add its values to ' \
+                           'N_rigid_modalities == \n'
 
         if len(self.propagation_options['Affine_reg_masks']) > 0:
             if not len(self.propagation_options['Affine_reg_masks']) == len(self.propagation_options['Affine_modalities']):
diff --git a/spot/tools/iterative_propagator.py b/spot/tools/iterative_propagator.py
index 3cab87f..487d853 100644
--- a/spot/tools/iterative_propagator.py
+++ b/spot/tools/iterative_propagator.py
@@ -140,7 +140,7 @@ def non_rigid_propagator(sp):
             for _ in range(num_modalities)]
         stack_a_list_of_images_from_list_pfi(pfi_target_reg_mask_list, pfi_target_reg_mask)
 
-    else:
+    else:  # TODO every input subject must have roi_mask and reg_mask per modality.
         assert len(sp.propagation_options['N_rigid_modalities']) == len(sp.propagation_options['N_rigid_reg_masks'])
         pfi_target_reg_mask_list = [
             jph(pfo_target_masks, '{0}_{1}_{2}.nii.gz'.format(sp.target_name, m, target_suffix_mask))