VBA Error 91

Good day to everyone-

I have a database that uses a userform to show information on my student testing scores. When I double click on some of the students names in the listbox named "lstLookup" I receive this error message ... "Runtime Error 91: Object variable or with block variable not set" (see code below). I only get the error on about half of the students. Can someone please help and explain why this happens and how to resolve it?

Thank you Charlie

HTML Code:

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Private Sub lstLookup_DblClick(ByVal Cancel As MSForms.ReturnBoolean)
    'declare the variables
    Dim cIDNumber As String
    Dim I As Integer
    Dim findvalue
    'error block
    On Error GoTo errHandler:
    'get the select value from the listbox
    For I = 0 To lstLookup.ListCount - 1
        If lstLookup.Selected(I) = True Then
            cIDNumber = lstLookup.List(I, 1)
        End If
    Next I
    'find the Id number
    Set findvalue = Sheet2.Range("F:F").Find(What:=cIDNumber, LookIn:=xlValues).Offset(0, -3)
    'add the database values to the userform
    cNum = 41
    For X = 1 To cNum
        Me.Controls("Reg" & X).Value = findvalue
        Set findvalue = findvalue.Offset(0, 1)
    Next
    'disable adding
    Me.cmdAdd.Enabled = False
    Me.cmdEdit.Enabled = True
    'error block
    On Error GoTo 0
    Exit Sub
errHandler::
    MsgBox "An Error has Occurred  " & vbCrLf & "The error number is:  " _
          & Err.Number & vbCrLf & Err.Description & vbCrLf & _
          "Please notify the administrator"
End Sub

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not sure, but maybe so

Code:

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....
'find the Id number
Set findvalue = Sheet2.Range("F:F").Find(What:=cIDNumber, LookIn:=xlValues).Offset(0, -3)
If findvalue Is Nothing Then MsgBox cIDNumber & " not found": Exit Sub' this line added
'add the database values to the userform
....

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Thank you Nilem for looking at my question. I inserted your code suggestion, but I still get Error 91 on some of the students.

Charlie

could you attach a file, in which you get an error

Put a break point on the first line and step through with a name that throws an error.

Identify the offending line