Using Null in a DLookup Criteria - ms-access

Question

I am creating a database to manage the flow of people in and out of an incident system. My idea is to use the DLookup function to allow me to identify a null value in the checkout date. This would allow me to prevent someone from jumping incidents without signing out first.
If it is possible I would like to get the EVENT ID and the COMMAND POST ID so I can create an error message to tell me the incident that is member is still attached to.
'***Section 1 - Ensure member is Checked out and DEMOBed from previous incidents
'******Variables
'*********Variable to hold the Member ID
Dim id As Integer
'*********Variable to hold the checkout status
Dim checkout As String
'*********Variable to hold the Event ID
Dim eventID As Integer
'******Code Block 1 - Check for Null Values
id = Me.Text18
Forms![frm_ics238Table].Refresh
If Not IsNull(DLookup("[eventID]", "[frm_ics238Table]", "[checkoutDate] is Null And employeeID = '" & Me.Text18 & "'")) Then
MsgBox "y"
End If
End Sub

Answer 1

From your original question which right now it doesn't look like you've edited it to indicate the updated code and errors, you had:
id = Me.Text18
Forms![frm_ics238Table].Refresh
If Not IsNull(DLookup("[eventID]", "[frm_ics238Table]", "[checkoutDate] is Null And employeeID = '" & Me.Text18 & "'")) Then
MsgBox "y"
End If
I think these might help:
"[frm_ics238Table]" should be table:"[tbl_ics238Table]"
ID appears to be declared as an integer yet you're using it as a
string. If employeeID is actually a numeric field remove the quotes
around employeeID = '" & Me.Text18 & "'" It should look like this employeeID = " & ID & "
You can also try using IsNull([checkoutDate]) instead of
[checkoutDate] is Null
SO it might look like:
If Not IsNull(DLookup("[eventID]", "[tbl_ics238Table]", "IsNull([checkoutDate]) And (employeeID = " & ID & ")")) Then
MsgBox "y"
End If
Even better would be to put the criteria into a string variable so you could debug and test it first
Dim strCriteria as String
strCriteria = "IsNull([checkoutDate]) And (employeeID = " & ID & ")"
Debug.Print strCriteria
If Not IsNull(DLookup("[eventID]", "[tbl_ics238Table]", strCriteria)) Then
MsgBox "y"
End If
EDIT: PROOF OF WORKING CODE:
Not that I don't believe you but I don't believe that it doesn't work if what you've described matches my assumptions.
I tested successfully using assumptions that:
eventID - number field
checkoutDate - date/time field
employeeID - number field
My sample data table
Then I put a command button on a form to test the code:
Private Sub Command8_Click()
Dim strCriteria As String
Dim ID As Integer
ID = 4
strCriteria = "IsNull([checkoutDate]) And (employeeID = " & ID & ")"
Debug.Print strCriteria
If Not IsNull(DLookup("[eventID]", "[tbl_ics238Table]", strCriteria)) Then
MsgBox "y"
End If
End Sub
Finally - testing the button click resulted in:
In conclusion - if you're still getting an error then you can update
your question with BOTH your code AND your table design.