diff --git a/python/damask/_grid.py b/python/damask/_grid.py
index 61b64e018..561f851be 100644
--- a/python/damask/_grid.py
+++ b/python/damask/_grid.py
@@ -1183,8 +1183,9 @@ class Grid:
                           np.in1d(stencil,np.array(list(set(selection) - {me}))))
 
         offset_ = np.nanmax(self.material)+1 if offset is None else offset
-        selection_ = util.tbd(selection) if not invert_selection else \
-                     list(set(self.material.flatten()) - set(util.tbd(selection)))
+        selection_ = util.tbd(selection)
+        if selection_ is not None and invert_selection:
+            selection_ = list(set(self.material.flatten()) - set(selection_))
         mask = ndimage.filters.generic_filter(self.material,
                                               tainted_neighborhood,
                                               size=1+2*vicinity,
diff --git a/python/damask/util.py b/python/damask/util.py
index 1588906a2..880e815e8 100644
--- a/python/damask/util.py
+++ b/python/damask/util.py
@@ -16,7 +16,7 @@ import numpy as np
 import h5py
 
 from . import version
-from ._typehints import FloatSequence, NumpyRngSeed
+from ._typehints import FloatSequence, NumpyRngSeed, IntCollection
 
 # limit visibility
 __all__=[
@@ -752,7 +752,7 @@ def tail_repack(extended: Union[str, Sequence[str]],
           list(extended[len(existing):]))
 
 
-def tbd(arg) -> List:
+def tbd(arg: Union[IntCollection,int,None]) -> Union[List,None]:
     if arg is None:
         return None
     elif isinstance(arg,(np.ndarray,Collection)):