in the 1.40rc1 release notes: https://pandas.pydata.org/pandas-docs/version/1.4/whatsnew/v1.4.0.html#null-values-are-no-longer-coerced-to-nan-value-in-value-counts-and-mode We are seeing distinct `NaN` , but I think we should be seeing `NaN` and `NaT` Looks like we patched here: https://github.com/pandas-dev/pandas/pull/42743