if (features[i] == null || features[i].length != nInArrays) {
                throw new IllegalStateException(
                        "Cannot merge MultiDataSets with different number of input arrays: toMerge[0] has "
                                + nInArrays + " input arrays; toMerge[" + i + "] has "
                                + (features[i] != null ? features[i].length : null) + " arrays");