Unable to get the FindNext property of the Range Class

I keep getting that error when I'm running my macro. When I debug, it points me to the bolded line in the code below.

The larger macro I'm running this function in runs this function some 101 times without error before this happens.

The values of the parameters are as follows when it gives me the error:
Find_Exact("hchen", ws1, "B:B")

The first parameter is the only one that changes in the previously mentioned running of this function.

Function Find_Exact(ByVal sText As String, ByRef ws As Worksheet, ByVal sRange As String) As Range
    
    Dim first_addr As Range
    Dim found_addr As Range
    Set first_addr = ws.Range(sRange).Find(sText, LookIn:=xlValues)
    If Not first_addr Is Nothing Then
        If ws.Cells(first_addr.Row, first_addr.Column) = sText Then
            Set Find_Exact = first_addr
        Else
            Do
                Set found_addr = ws.Range(sRange).FindNext(sText)
                If Not found_addr Is Nothing Then
                    If ws.Cells(found_addr.Row, found_addr.Column) = sText Then
                        Set Find_Exact = found_addr
                        Exit Do
                    End If
                End If
            Loop While Not found_addr Is Nothing And found_addr <> first_addr
        End If
    End If
End Function

What could possibly be the problem??

Thanks for the help.

Maybe this:

Function FindExact(sText As String, _
                   wks As Worksheet, _
                   sRng As String) As Range

    Set FindExact = wks.Range(sRng).Find(What:=sText, _
                                         LookIn:=xlValues, _
                                         LookAt:=xlWhole, _
                                         MatchCase:=True, _
                                         SearchFormat:=False)
End Function

Does xlWhole guarantee that cell1.value = cell2.value? What does search_format specify?

Help is your friend.