SSRS calculated field strange behavior
It is because it may not have a value at that pariticular field/place so first replace the value providing box with:



iif(IsNothing(sum(field)),"0",(sum(field))


Then replace row summary field with the below expression:

=iif(cInt(ReportItems!textbox1name.value)=0,100,(ReportItems!textbox2.value/iif(cInt(ReportItems!textbox1name.value)=0,100,ReportItems!textbox1name.value)*100))