Trac #21204: cherry-pick to avoid merge conflict

Author: dkrenn

src/sage/combinat/k_regular_sequence.py

@@ -883,7 +883,7 @@ def _n_to_index_(self, n):
         try:
             return W(n.digits(self.k))
         except OverflowError:
-            raise ValueError('value {} of index is negative'.format(n))
+            raise ValueError('value {} of index is negative'.format(n)) from None
 
     def guess(self, f, n_max=100, max_dimension=10, sequence=None):
         r"""