help on error message

i got this code that when i get to put a value in room service the whole thing stop working and tell me Conversion from string "" to type 'Double' is not valid."}. my code is suppose to calculate subtotal which is telephoneChg + RoomService + Laundry my code is the follow please help

If TextBox4.Text.Length = 0 AndAlso TextBox5.Text.Length = 0 AndAlso TextBox6.Text.Length Then Exit Sub
If Not IsNumeric(TextBox4.Text) OrElse Not IsNumeric(TextBox5.Text) AndAlso TextBox6.Text.Length Then Exit Sub
 
        dblRoomService = CDbl(TextBox5.Text)
        dblTelephone = CDbl(TextBox4.Text)
        dblLaundry = CDbl(TextBox6.Text)
        Subtotal = dblRoomService + dblTelephone + dblLaundry
        TextBox7.Text = CStr(Subtotal)
        TextBox7.Text = Format(Subtotal, "$#,##0.00")
    End Sub

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