Reporting Services :: SSRS Matrix - Add A Column In A Matrix With A Variance

I got the following code to add a column in a matrix with a variance:

IIF(IsNothing(Previous(Sum(Fields!Amount.Value))) or Fields!year.Value=First(Fields!year.Value,"Category") or Previous(Sum(Fields!Amount.Value))=0,nothing,
(
(Fields!Amount.Value)
/Previous(sum(Fields!Amount.Value))
)
)

This code works fine, except that the first row of the matrix shows an #error

This happens with each matrix where I use this expression. A warning emerges:

rsruntimeerrorinexpression the value expression for the textrun Textbox43.Paragraphs[0].TextRuns[0]' contains an error.

Attempted to divide by zero.

The strange thing is that the part

Fields!year.Value=First(Fields!year.Value,"Category")
should prevent an error and I expect it to show 'nothing'

An screenshot of the table. (each color is a different category. Each row stands for 2013, 2014, 2015)

As you can see, all other 2013 rows show a blank cell, except the first row.