You do not state the context of the query (SSAS cube design calculations tab, SSMS query or SSRS report dataset)

----- SSAS Scoped assignment

DECLARE MEMBER CURRENTCUBE.[Measures].[PercentRoot-Scoped]

AS "N/A", VISIBLE = 1, FORMAT_STRING="0.00%";

SCOPE ([Measures].[PercentRoot-Scoped], [Geography].[City].[City].MEMBERS);

THIS = ([Geography].[City].CURRENTMEMBER, [Measures].[Internet Sales Amount]) /

(ROOT([Geography]),[Measures].[Internet Sales Amount]);

END SCOPE;

----- SSMS and SSRS

WITH MEMBER [Measures].[PercentOfRoot] AS

([Geography].[City].CURRENTMEMBER, [Measures].[Internet Sales Amount]) /

(ROOT([Geography]),[Measures].[Internet Sales Amount]),

FORMAT_STRING = '0.00%'

SELECT

{[Measures].[Internet Sales Amount], [Measures].[PercentOfRoot]} ON COLUMNS,

NON EMPTY [Geography].[City].[City].MEMBERS ON ROWS

FROM

[Sales Cube]

It may be that the ROOT is not available (isaggregateable?) or that the ROOT value is NULL (calculation assignment?).

Fitz