Fdr threshold is inf

I’m trying to use FDR thresholding in nilearn

z_map = second_level_model.compute_contrast(output_type='z_score')

(z_map contains values ranging from -3.5 to 4.6)

thresholded_map2, threshold2 = threshold_stats_img(z_map, alpha=.05, height_control='fdr')

>> threshold2

And I get this warning:
param_validation.py:72: UserWarning: The given float value must not exceed 4.436272. But, you have given threshold=inf

What could be the reason for this?