[AccessD] Null in table and code

[Kaup, Chester]

I have a field named oil in a table that is defined as numeric single. In some records this field is null. I have some code in which I have a variable defined as a single. When I try to write a field that is null from the table to the variable I get a message of invalid use of a null. How can a single in a table hold a null but a variable defined as a single in code not contain a null?