Adding Data Bound Image to Header Reporting Services SSRS

I am attempting to add a data-bound image to report headers in Reporting Services.

I understand that I need to place a textbox in the body of the report and then, using some conversion expressions, bring the image up into the header. I have done this using the instructions below, but I continue to get #Error in the header. I know the =Convert.ToBase64String(Fields!Photo.Value) function is working, but apparently the second conversion is failing.

Any advice on this?

Instructions for Adding a Data-Bound Image to a Header or Footer:

You can use image data stored in a database in a header or footer. However, you cannot reference database fields from the Image report item directly. Instead, you must add a text box in the body of the report and then set the text box to the data field that contains the image (note that the value must be base64 encoded). You can hide the text box in the body of the report to avoid showing the base64-encoded image. Then, you can reference the value of the hidden text box from the Image report item in the page header or footer.

For example, suppose you have a report that consists of product information pages. In the header of each page, you want to display a photograph of the product. To print a stored image in the report header, define a hidden text box named TXT_Photo in the body of the report that retrieves the image from the database and use an expression to give it a value:

=Convert.ToBase64String(Fields!Photo.Value)

In the header, add an Image report item which uses the TXT_Photo text box, decoded to show the image:

=Convert.FromBase64String(ReportItems!TXT_Photo.Value)