Mathematica 11.3.0.0 Windows 10 64
Before posting a bug, I would like if anybody can verify:
The following works o.k:
Sum[Times @@ (IntegerDigits[i, 7] + 1), {i, 0, 10^6}]
However if the Summand is a function we get some gibberish:
fu[n_] := Times @@ (IntegerDigits[n, 7] + 1);
Sum[fu[i], {i, 0, 10^6}]
Interestingly, the following works:
Sum[fu[i], {i, 1, 10^6}]