helpful error message

python/damask/_crystal.py

@@ -1201,9 +1201,12 @@ class Crystal():
 
         sep = '-->'
         search = self.lattice+sep+('' if target is None else target.lattice)                        # type: ignore
-        m_l,o_l = [transform.split(sep) for transform in orientation_relationships[model].keys()   # type: ignore
-                   if transform.startswith(search)][0]
+        transform = [t for t in orientation_relationships[model].keys() if t.startswith(search)]    # type: ignore
 
+        if len(transform) != 1:
+            raise ValueError(f'invalid target lattice "{search.split(sep)[1]}"')
+
+        m_l,o_l = transform[0].split(sep)                                                           # type: ignore
         m_p,o_p = orientation_relationships[model][m_l+sep+o_l]
         other = Crystal(lattice=o_l) if target is None else target
         m_p = np.stack((self.to_frame(uvw=m_p[:,0] if len(m_p[0,0])==3 else util.Bravais_to_Miller(uvtw=m_p[:,0])),

python/tests/test_Crystal.py

@@ -110,3 +110,11 @@ class TestCrystal:
         for r in crystal.orientation_relationships:
             crystal.relation_operations(r)
 
+    @pytest.mark.parametrize('crystal', [Crystal(lattice='cF'),
+                                         Crystal(lattice='cI'),
+                                         Crystal(lattice='hP')])
+    def test_related_invalid_target(self,crystal):
+        relationship = np.random.choice(crystal.orientation_relationships)
+        with pytest.raises(ValueError):
+            crystal.relation_operations(relationship,crystal)
+