That's very odd - that's in the run of deltas (changes in position) for a morph, and there's no obvious reason for it to fail there. I do wonder if perhaps the number of deltas line is wrong, earlier in the file, and that's the point at which the index (the first number on the line) exceeds the declared number. If you search backwards from there for

deltas

you should find a block like this

            indexes #
            numbDeltas #
            deltas
                {

where # will be some number - what actual values do you have?