Rewrite Parameter not editable after creation.

I'm importing a CSV file with no headers. Source mapping fields have the form:

[0]
[1]
[2]
[...]
[10]
[11]

I've got a replacement plugin with the Rewrite Parameter:

[10][11]

This does what I want it to do ( It concatenates fields 10 and 11 ), but editing a rewrite plugin item results in:

warning: Invalid argument supplied for foreach() in <removed>\sites\all\modules\feeds_tamper\plugins\rewrite.inc on line 18.

In addition to the problem on line 18, it looks like $form['text'] has no #default_value so existing replacement patterns will not make it to the form.