Description
Event (Overdispersed Poisson) Areal Interpolation assumes that a particular event was counted over a designated polygon for a specified amount of time. For each polygon, the count field specifies the number of events witnessed in that polygon, and the time field specifies the amount of time spent observing. Thus, the count field cannot be negative (it can be zero if no events were observed), and the time field must be positive.
Solution
In the Attribute Table of the polygon feature class, sort both the count and time fields to make sure that neither contains negative values (and that time is not zero).