s2rlgc produces negative values in R matrix

My experiments showed that w/ most snp files, the s2rlgc produces negative R values at high frequencies. Those snp files were created by varous sources such as ADS, HFSS, etc. Does this sound like a bug?

[Jason]

For non-uniform transmission lines or transmission lines that have unwrapped phase exceeding -180 deg, you can expect strange behavior when extracting RLGC parameters.

[Sherman Chen]

Yes, that's the reason. I figured it out later on. Thanks for the comments.