Nan value of Mean in histogram

I have a question about one of my drawn histograms.
What’s the reason for nan value of Mean as you see? What I understand from the shape of this hist, the mean must be around -0.3, but I don’t know why it shows Nan value to me.
Hist has been drawn using TH1D class.

Thanks in advance

ROOT Version: 6.22

When filling the histogram, make sure that you do not try to fill (some bins) with “nan”.

You could also try now: h_cos1ee->ResetStats();