Probleme mit UBound und LBound

Public Function Interp1(xArr As Variant, yArr As Variant, x As Double) As Double
If ((x  xArr(UBound(xArr)))) Then
MsgBox "Interp1: x is out of bound"
Exit Function
End If
If xArr(LBound(xArr)) = x Then
Interp1 = yArr(LBound(yArr))
Exit Function
End If
Dim i As Single
For i = LBound(xArr) To UBound(xArr)
If xArr(i) >= x Then
Interp1 = yArr(i - 1) + (x - xArr(i - 1)) / (xArr(i) - xArr(i - 1)) * (yArr(i) - yArr(i - 1) _
_
)
Exit Function
End If
Next i
End Function