diff --git a/python/MRzeroCore/phantom/voxel_grid_phantom.py b/python/MRzeroCore/phantom/voxel_grid_phantom.py
index ccd3382..f5ebebb 100644
--- a/python/MRzeroCore/phantom/voxel_grid_phantom.py
+++ b/python/MRzeroCore/phantom/voxel_grid_phantom.py
@@ -164,6 +164,9 @@ def dephasing_func(t, n): return sigmoid(t, n)
             def dephasing_func(t, _): return identity(t)
         else:
             raise ValueError(f"Unsupported voxel shape '{voxel_shape}'")
+        
+        if not self.tissue_masks:
+            self.tissue_masks = {"combined": mask}
 
         return SimData(
             self.PD[mask],
@@ -180,8 +183,8 @@ def dephasing_func(t, _): return identity(t)
             dephasing_func,
             recover_func=lambda data: recover(mask, data),
             phantom_motion=self.phantom_motion,
-            voxel_motion=self.voxel_motion,
-            tissue_masks=self.tissue_masks
+            voxel_motion=self.voxel_motion,            
+            tissue_masks = self.tissue_masks
         )
     
     @classmethod