SSRS Expression error when uploading report

It would seem the daemons are out to get me on this one :) every time I think Im there I get another error. This time is when I try to upload the report to our SSRS server. It errors with

The Value expression for the textrun ‘users_text02.Paragraphs[0].TextRuns[0]’ contains an error: [BC30451] Name 'W391' is not declared. (rsCompilerErrorInExpression) 

If I delete the textbox its referring to the report uploads fine. The expression in the text box is an IIF statement as below

=IIF(Fields!supplier.Value="W391",Fields!users_text05.Value,Fields!users_text02.Value)

Any ideas? Thanks