sanity check for shift

Author: fchapoton

src/sage/rings/polynomial/integer_valued_polynomials.py

@@ -227,7 +227,7 @@ def shift(self, j=1):
                     B[3] + B[4]
                 """
                 A = self.parent()
-                return A._from_dict({i + j: c for i, c in self})
+                return A._from_dict({A._indices(i + j): c for i, c in self})
 
             def sum_of_coefficients(self):
                 """