How do I check for the top level in a dimension to avoid division by an nonexsting parent(-1.#INF)

 How do I check for the top level in a dimension to avoid division by an nonexsting parent(-1.#INF)

Query Example:

WITH  Member Measures.ParentRatio As IIF([Product].[Product Categories].CurrentMember.Parent IS NULL, Null,
[Measures].[Internet Sales Amount]/
([Measures].[Internet Sales Amount],[Product].[Product Categories].CurrentMember.Parent)), Format_String = '###.#%'
Select {[Measures].[Internet Sales Amount],Measures.ParentRatio} On Columns,
NON EMPTY Descendants([Product].[Product Categories],[Product].[Product Categories].[Subcategory],Self_And_Before) On Rows
From [Adventure Works]Where [Date].[Calendar].[Calendar Year].&[2003];

Result will be:

	Internet Sales Amount	ParentRatio
All Products	$9,791,060.30	(null)
Accessories	$293,709.71	3.00%
Bike Racks	$16,440.00	5.60%
Bike Stands	$18,921.00	6.40%
Bottles and Cages	$23,280.27	7.90%
Cleaners	$3,044.85	1.00%
Fenders	$19,408.34	6.60%
Helmets	$92,583.54	31.50%
Hydration Packs	$16,771.95	5.70%
Tires and Tubes	$103,259.76	35.20%
Bikes	$9,359,102.62	95.60%
Mountain Bikes	$3,989,638.48	42.60%
Road Bikes	$3,952,029.21	42.20%
Touring Bikes	$1,417,434.93	15.10%
Clothing	$138,247.97	1.40%
Caps	$7,956.15	5.80%
Gloves	$14,228.69	10.30%
Jerseys	$70,370.46	50.90%
Shorts	$30,445.65	22.00%
Socks	$2,229.52	1.60%
Vests	$13,017.50	9.40%