Shell (?) problem with procmail

I have encountered a problem when I moved a procmailrc file
to a new linux system. I suspect it is to do with the shell
since the same file has worked under both SunOS and another
Linux system.

The following formula:

:0
| ( formail -A"X-PIM-Maintenance-No: `echo $COUNTER` " -I "To: \
nils(_at_)propertymall(_dot_)com" ; \
echo "Order PIM-MAINTENANCE " $COUNTER " assigned"; ) | $SENDMAIL -t   


creates the following error:


procmail: Executing " ( formail -A"X-PIM-Maintenance-Order-No: `echo $COUNTER` "
 -I "To: nils(_at_)propertymall(_dot_)com" ; \
        echo "Order PIM-MAINTENANCE " $COUNTER " assigned"; ) | $SENDMAIL -t"
if: Badly formed number.
Illegal variable name.        

Clearly there is a problem in that the attempted pipe into the formail
is not recognised properly...

...but what can I do to solve this?

Any help appreciated!

Thanks,

Nils
nils(_at_)propertymall(_dot_)com