I managed to solve the problem.

I changed one of the false statements to 1 instead of 0, the Nan-value will disappear.

Ex:

=Sum(IIf(Fields!Name.Value="Stock",Fields!RiskValue.Value,0),"DS_AM_GetRisk_AssetType")/ Sum(IIf(Fields!Name.Value="Stock",Fields!GlobalMarketValue.Value,1),"DS_AM_GetRisk_AssetType")

hope this can be helpful for someone else!