Invalid use of Null when using DLookup in MS Access - vba

Question

I want to set the user login access in MS Access which means that if the user logs in as the admin it will show a different form.
I have tried to get the userlevel which is a string and will show things like "Admin" or "User" but it indicated:
Invalid use of Null
At this line:
UserLevel = DLookup("UserSecurity", "tblUser", "[UserLogin] = ' " & Me.txtLoginID.Value & "'")
Here is the full code:
Private Sub Command1_Click()
Dim UserLevel As String 'get the dlookup value
Dim TempPass As String
If IsNull(Me.txtLoginID) Then
MsgBox "Please Enter Login ID", vbInformation, "Login Id Required"
Me.txtLoginID.SetFocus
ElseIf IsNull(Me.txtLoginPass) Then
MsgBox "Please Enter Password", vbInformation, "Login password Required"
Me.txtLoginPass.SetFocus
Else
'process the job
If (IsNull(DLookup("UserLogin", "tblUser", "UserLogin ='" & Me.txtLoginID.Value & "'"))) Or _
(IsNull(DLookup("password", "tblUser", "Password = '" & Me.txtLoginPass.Value & "'"))) Then
MsgBox "Incorrect Password"
Else
TempPass = DLookup("password", "tblUser", "UserLogin = '" & Me.txtLoginID.Value & "'")
UserLevel = DLookup("UserSecurity", "tblUser", "[UserLogin] = ' " & Me.txtLoginID.Value & "'")
'get the usersecurity whcih indicate he is admin or user
DoCmd.Close
If UserLevel = "Admin" Then 'if admin then open employee form else open customer form
'MsgBox "Login ID and password correct "
DoCmd.OpenForm "Employee"
Else
DoCmd.OpenForm "CustomerForm"
End If
End If
End If
End Sub
I have tried to use Nz() but it gives me a null value which takes me to the customer form.

Answer 1

To explain the error that you are receiving: this arises when you attempt to assign a Null value to a variable whose data type is not a Variant, per the MS documentation:
A Variant is a special data type that can contain any kind of data [...] A Variant can also contain the special values Empty, Error, Nothing, and Null.
This error arises in your code because the DLookup function will return Null when no records in the domain fulfill the supplied criteria argument, and can be boiled down to the following two lines:
Dim UserLevel As String
UserLevel = DLookup("UserSecurity", "tblUser", "[UserLogin] = ' " & Me.txtLoginID.Value & "'")
I suspect that this is caused by the leading space in your criteria argument:
"[UserLogin] = ' " & Me.txtLoginID.Value & "'"
^--------------------------------- HERE
Which should probably be:
"[UserLogin] = '" & Me.txtLoginID.Value & "'"
However, you may still wish to account for the case in which no records meet the criteria, which can be accomplished in several ways.
You could use the Nz function and then test for an empty string, e.g.:
UserLevel = Nz(DLookup("UserSecurity", "tblUser", "[UserLogin] = '" & Me.txtLoginID.Value & "'"), "")
Select Case UserLevel
Case "Admin": DoCmd.OpenForm "Employee"
Case "User" : DoCmd.OpenForm "CustomerForm"
Case Else : MsgBox "Invalid UserSecurity Value"
End Select
Or, you could define the UserLevel variable as a Variant (hence permitting a Null value), and test whether such variable is Null using the IsNull function:
Dim UserLevel As Variant ' Or just "Dim UserLevel" since Variant is the default type
UserLevel = DLookup("UserSecurity", "tblUser", "[UserLogin] = '" & Me.txtLoginID.Value & "'")
If IsNull(UserLevel) Then
MsgBox "Invalid UserSecurity Value"
ElseIf UserLevel = "Admin" Then
DoCmd.OpenForm "Employee"
Else
DoCmd.OpenForm "CustomerForm"
End If

Answer 2

Remove the space you have inserted in your criteria, so:
UserLevel = DLookup("UserSecurity", "tblUser", "[UserLogin] = '" & Me.txtLoginID.Value & "'")