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

Error:

Invalid Use of Null Alert

Cause:

The Eval() function is used on a field that does not contain data. The Eval() function evaluates an expression that results in a text string or a numeric value, therefore requires fields with data. This is by design of Microsoft Access.

Solution:

Use the Eval() function only on fields with data.