If you give a non-existent key to the subset argument it will default to the last column. For example:
In [3]: d = pd.DataFrame({'a':[1],'b':[2],'c':[np.nan]})
In [4]: len(d.dropna(subset=['x']))
Out[4]: 0
It seems the problem is in the line where dropna takes the subset:
agg_obj = self.take(ax.get_indexer_for(subset),axis=agg_axis)
Here, ax.get_indexer_for(subset) will return -1 as a sentinel value for any keys that were not found in subset, and self.take interprets the -1 as a request for the last column.
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