SSRS MultiLookup error - SQL Server 2008 R2

I have a report that contains 2 DataSets.

DataSet1 looks like this:

SRId SRName RelatedItemIdList
1        A            1,2,3
2        B            1
3        C            NULL

DataSet2 looks like this:

RelatedItemId  ItemName
1                      Item1
2                      Item2
3                      Item3

I am using MultiLookUp to display the ItemName in one of the report columns. I added this expression to the Textbox:

=Join(MultiLookup(Split(Fields!RelatedItemIdList.Value,","),Fields!RelatedItemId  Key.Value,Fields!ItemName.Value,"DataSet2"),", ")

This shows #Error in the textbox when i run the report.

What am i doing wrong here?