Error: Invalid Use of Null (due to using Val on a field with null value).

Error:

Invalid Use of Null

Cause:

The Val() function is used to evaluate a field that does not contain valid data (a field with a null value). The Val() function returns the numbers contained in a string, therefore requires valid data. This is by design of Microsoft Access.

Solution:

Use the Val() function to evaluate only fields that have valid data.