Issue with logfoldchanges in scanpy.tl.rank_genes_groups

Hello, I am having problems with the logfoldchanges when running scanpy.tl.rank_genes_groups in my single cell RNA sequencing analysis. I am getting the following error:

RuntimeWarning: invalid value encountered in log2 self.stats[group_name, ‘logfoldchanges’] = np.log2(

Then when I try to see the values they are all nan. Your help is much appreciated. Thank you.