Simple Cast not working

[FastEddie]

I cannot figure out what I am doing wrong. This is a very simple cast:

'sng casting test
        Dim varFF As Single = CSng(tbCalcFF.Text) - CSng(tbGoalFF.Text)
        If varFF < 0 Then
            tbCalcFF.BackColor = Color.Gold
        Else
            tbCalcFF.BackColor = Color.LightGoldenrodYellow
        End If

I am getting this error: Conversion from string "" to type 'Single' is not valid.