Make sure the criterion portion of DLookup is a valid expression.  For instance, if your criterion is "[MyField]=" & Me!MyField, then if you delete the field's value on the form, the expression becomes "[MyField]=" , which is invalid and causes an error.

How do I make it a valid expression? Can I create a blank option in the combo box so that instead of deleting the entry, I can have my users insert a blank entry instead?

Try '[MyField]=' & Nz(Me!MyField,0)

I'm guessing your field in question is numeric, because that is what usually causes the erroneous criterion "[MyField]=" : when the numeric field becomes null.

Try this as the criterion for DLookup:

"[MyNumericField] & '' = " & Me!MyNumericField & " & ''"

Note carefully the single quotes and double quotes used above.

What this does is, it converts both sides to strings.  If the field value on the right side is a null, it is turned into a zero-length string so it can be validly compared to the string on the left side.  If the user types spaces or even gibberish text in the field (assuming you don't validate user entries), it still works.  That's because it compares string to string, and strings can be any text, unlike numerals.

If this still doesn't work for you, post your exact DLookup function.

Liam, that did the trick. Thank you.

Kevin, I can't get your criterion to work. I think I'm missing where to put the closed parenthesis from the beginning of the function =DLookup(
Where do I close the parenthesis? I'm guessing that is impacting the result. I put it at the very end and it didn't work.
Also, what does Me! represent? Is that just saying you're referencing something on that form or does that need to be typed into the function?
Last point of clarity - the brackets. I know they represent the field name, but does that effect the outcomes of these functions whether I have them or not?

Thank you for the knowledge!

Hi Daniel, glad to see you worked it out.  Yes, you need the closing parenthesis for all functions, e.g. Dlookup(argument 1, argument 2, argument 3).  These long functions and commands are tricky, and easy to make mistakes in even for experienced users.

"Me" standards for the current form.  Writing Me!MyField makes it clear that MyField is a control on the current form.  Writing only MyField is fine too, but if you have very long code, it helps to differentiate them.  The exclamation point "!" means "is a parent of"  Newer generations of users also use period "." instead "!" to conform more to the naming conventions of the object-oriented system; e.g. Me.MyForm.

Square brackets are needed if your field names have spaces: e.g. [My field].  Yes, it is best to have field names with no spaces.  But I always use brackets in my examples to make sure they work for the user, rather than assuming the user doesn't have spaces in their field names.