There only 37 head sprites.
I'm hoping you don't mean that the head sprite file only includes 37 images? 0.o; If that's the case, then that is definitely your problem.
The game expects a head sprite file to include 192 images; it cannot anticipate what it is supposed to do if it does not have the correct number, all it knows is that the conditions it was programmed to operate under have not been met, so it crashes. Unless you want to go in and change the way the game reads the sprite file, which I'm not even sure is possible, you need to have the correct number of images.
The game expects that a head will have four sprites facing right, at different angles; then four facing left; then four facing front; then four facing back; then all of these repeated but with the eyes closed; then all of that done over again for the next expression, and then the next, and the next, and so on. If you don't want to make this many images, I would recommend using copies of those images you have made that will look best in place of those you're leaving out. For example, if you only want your Ettin to stare at head level, without looking up or down, you should copy the head-level sprites and put those copies where the other sprites you didn't make would have gone.