min and char functions

I am having problems with a formula
I have columns each containing a letter grade eg ABCD
I have a formula that will return the maximum or minimum garde in the column that is
=char(min(code(a1),code(b1),code(c1),code(d1)))
This works fine until I have unfilled/blank colums eg A_CD (for missing assignment grades) or A ___ (only first assignment complete)
As soon as this is the case I just get #VALUE! error.

Can anyone help?

Thank you in anticipation
Mark

PS I would also like to use the MODE function on cells that contain a grade

The information you have provided is not enough for anyone to offer you help. Please help us to help you by providing appropriate information.

Try

=CHAR(IF(LEN(A1:A7)=1,MAX(CODE(A1:A7))))
Entered as an Array (use CNTRL SHFT ENTER instead of ENTER)
Does that work for you?