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