[exim] Weird - possible bug?

Transport looks like this and works normally:

process_and_forward_smtp:
driver = smtp
delivery_date_add
interface = $acl_c_smtp_out
helo_data = SERVER_NAME.junkemailfilter.com

However - if I set a variable:

set acl_c_helo_data = SERVER_NAME.junkemailfilter.com

and then use this transport:

process_and_forward_smtp:
driver = smtp
delivery_date_add
interface = $acl_c_smtp_out
helo_data = $acl_c_helo_data

I get this:

2012-05-28 17:36:05 rejected EHLO from apn-outbound.junkemailfilter.com
[184.105.182.205]: syntactically invalid argument(s): (no argument given)

But I only get that sometimes. It is as if the variable forgets the
data. What am I missing? Thanks in advance.