TDT: data unbalanced causing dprime.nii to be null

My experimental design includes two conditions, in which the trial number in one condition is more than that in the other, resulting in data unbalanced. We want to perform ‘cross-classification’ through setting ' = ‘ok’ ',although the TDT analysis was completed smoothly, the dprime image of the analysis result was null, so that further analysis could not be made.I want to ask why does unbalanced data cause TDT to calculate the dprime as null? How to deal with unbalanced data in TDT?