[exim] $spam_report not working

I'm having a problem putting the $spam_report into a header:

warn    add_header = X-Spam-Score: $spam_score ($spam_bar)
    add_header = X-Spam-Report: $spam_report
    add_header = X-Spam-Score-Int: $spam_score_int

So - what am I doing wrong? Thanks in advance.