got segmentation error when the number of partitions is large

hi, I am using Metis5.0 to partition an unweighted graph of 7842 nodes and 315276 edges. It works fine for small number of partitions. But I got segmentation error when the number of partition is larger than 20. Can anyone help me on this? I checked some of the related posts of similar problem. I don't think its due to integer overflow as my graph doesn't even have weight. Thanks.