Some old versions of SML/NJ contained a bug in the Basis in which the large word type was 32 bits instead of 64 bits. This was fixed in 110.89, but older versions are still in use in some places.
The best solution to this problem is to update SML/NJ. If that is not possible, the following workaround may fix the problem. Starting from the Istari root directory:
cd cmlibi
cp convert-word.sml convert-word-nj64.sml
cp convert-word-nj32.sml convert-word.sml
rm -f cmlib.ipds
Then build again.