Invalid use of null in dlookup

I keep getting an error message saying invalid use of null... please help.

Dim dSubject As String
Dim dResponsibility As String
dSubject = DLookup("designsubject", "tblpart", "isnull(DesignRespond) And DateDiff('d', DesignStartDate, Now()) <= 3")
dResponsibility = DLookup("designresponsibility", "tblpart", "DesignRespond Is Null And DateDiff('d', DesignStartDate, Now()) <= 3")
If Not IsNull(dSubject) Then
    DoCmd.SendObject acSendNoObject, , acFormatHTML, "rgrguric@yahoo.com", , , "Employee has not responded for DESIGN responsibility, please review!!!", "User " & dResponsibility & " has not responded for DESIGN responsibility on " & dSubject & ".", False
End If

Thanks,
rgrguricAsked:
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Guy Hengel [angelIII / a3]Connect With a Mentor Billing EngineerCommented:

Dim dSubject As variant
Dim dResponsibility As variant
dSubject = DLookup("designsubject", "tblpart", "DesignRespond is null And DateDiff('d', DesignStartDate, Now()) <= 3")
dResponsibility = DLookup("designresponsibility", "tblpart", "DesignRespond Is Null And DateDiff('d', DesignStartDate, Now()) <= 3")
If Not IsNull(dSubject) Then
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Guy Hengel [angelIII / a3]Billing EngineerCommented:

Dim dSubject As String
Dim dResponsibility As String
dSubject = DLookup("designsubject", "tblpart", "DesignRespond is null And DateDiff('d', DesignStartDate, Now()) <= 3")
dResponsibility = DLookup("designresponsibility", "tblpart", "DesignRespond Is Null And DateDiff('d', DesignStartDate, Now()) <= 3")
If Not IsNull(dSubject) Then
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rgrguricAuthor Commented:
I put that in and I'm still receiveing the error same error message.
0
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Guy Hengel [angelIII / a3]Billing EngineerCommented:
explanation: only VARIANT and OBJECT data types can get a NULL value, string, integer etc cannot (will raise the error you have)
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rockiroadsCommented:
wrap DLOOKUP with ""


Dim dSubject As String
Dim dResponsibility As String


dSubject = NZ(DLookup("designsubject", "tblpart", "DesignRespond is null And DateDiff('d', DesignStartDate, Now()) <= 3"),"")
dResponsibility = NZ(DLookup("designresponsibility", "tblpart", "DesignRespond Is Null And DateDiff('d', DesignStartDate, Now()) <= 3"),"")

Now u can check for empty string

if dSubject = "" then
    msgbox "No Subject"
endif


U can use NZ to give a default value instead of "" instead
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rockiroadsCommented:
wrap DLOOKUP with NZ is what Im supposed to have said, oops
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