Variable Number Format

I'm trying to make math_percent be a specific format so I can use it in an e-mail later. However, this part of of the code doesn't work, math_percent.NumberFormat = "0%, why is that? I get an Object Required error, but I don't know what object I need and how to find out.

 Sub Whatver()
    Dim math_percent As Variant
    Dim wk9 As Worksheet
    Set wk9 = ThisWorkbook.Worksheets("Scorecard")
    math_percent = wk9.Range("G13").Value
    math_percent.NumberFormat = "0%"
 End Sub

Thanks for your help.