Problems with Rollapplyr function due to NA values (that should not be removed) in R

Question

I have a dataframe:

      date comp  ei
1   1/1/73    A  NA
2   1/4/73    A 0.6
3   1/7/73    A 0.7
4  1/10/73    A 0.9
5   1/1/74    A 0.4
6   1/4/74    A 0.5
7