Variable not set?

[bstubbs]

I've got the following code:

Function convertDriverName(driverName)
    Dim c As Range
    With Worksheets("lookup data").Range("a1:a500")
        Set c = .Find(driverName, LookIn:=xlValues, LookAt:=xlWhole)
        If Not c Is Nothing Then
            driverName = c.Offset(0, 2)
        End If
    End With
    convertDriverName = driverName
End Function

Sub findDrivers()
    Dim driverName, firstAddress, searchStr As String
    Dim r As Range
    Range("k2:k10000").Clear
    searchStr = "Employee:"
    Application.Calculation = xlCalculationManual
    With Worksheets("raw data - after").Range("d1:d10000")
        Set r = .Find(searchStr)
        If Not r Is Nothing Then
            firstAddress = r.Address
            Do
                r.Offset(-2, 7) = r.Offset(0, 1)
                Set r = .FindNext(r)
            Loop While Not r Is Nothing And r.Address <> firstAddress
        End If
    End With
    
    searchStr = "Driver:"
    With Worksheets("raw data - after").Range("d1:d10000")
        Set r = .Find(searchStr)
        If Not r Is Nothing Then
            firstAddress = r.Address
            Do
                driverName = r.Offset(0, 1)
                driverName = convertDriverName(driverName) ' <== no problem if commented out
                r.Offset(-2, 7) = driverName
                Set r = .FindNext(r)
            Loop While Not r Is Nothing And r.Address <> firstAddress
        End If
    End With
    Application.Calculation = xlCalculationAutomatic
    Range("a1").Select
    MsgBox ("Process complete...")
End Sub

Everything works fine if I have the call to the convertDriverName function commented out (see <== line, above). However, if the line is not commented out, I got an "Object variable or With block variable not set" error. Maybe I'm too close to the error, but I can't see what the problem may be. Can anyone help?

TIA

Bob