Except both possible test case errors

pyam/utils.py

@@ -152,7 +152,7 @@ def strip_R_integer_prefix(c):
                 except ValueError:
                     # nope, not an int, fall down to final return statement
                     pass
-        except TypeError:
+        except (TypeError, IndexError):
             # not a string/iterable/etc, fall down to final return statement
             pass
         return c