why are you storing numeric values in a varchar column. As experienced, this will get you in troubles.

( in most cases because of formatting issues or just because someone put non-numeric data in it)

There must be a zero ( 0 ) in your t1.B column.

add a case statement to check it so it doesn't produce the error.

select t1.a / case t1.b when 0 then NULL /* to the result will be null */

else t1.B

end as TheResult