You have specified a field for the self-potential parameter that includes some null values. Null values will be interpreted as zero for analysis.
Investigate the null values to make sure that interpreting them as zero is what you intended. If not, replace the null values or remove the features associated with the null values from the analysis. You can remove records by either physically removing them (edit the feature class and delete the features) or creating an active selection set that excludes these features.