I face a problem in detecting bold data in a shape

ActiveSheet.Shapes("Text Box 17").Select
MsgBox Selection.Characters(Start:=i, Length:=1).Font.FontStyle

when i use this similiar code to detect bolded character in a cell, it works
fine.
but when i use this code to detect bolded data in a text box or rectangle
shape
the error " invalid use of null message" appear

how to check bolded data in a shape in Excel 2000