The other common cause of this is (quoting from Microsoft Knowledgebase article Q190074
<hr>You have code behind a subform control that references a Boolean control such as
a check box on the main form, and are using an If..Then statement to implicitly
test for a value of True, as shown in the following example:
If Me.Parent![CheckBox] Then
The fix for that problem is to do one of two things: either change the If Then to explicitly test for a value of true, i.e., "If Me.Parent![CheckBox]=True Then." That's the preferred method. A cheat is to put the reference to the control in parens, which forces it to be evaluated.