Spurious FAIL reported from a WHILE loop

I have the following piece of template:

[% i = 0 %]
[% WHILE ( part_numbers.$i.defined || part_names.$i.defined ) %]
    <do stuff>
    [% i = i + 1 %]
[% END %]

This triggers the following FAIL:

FAIL	  filters
		missing_filter at line 12 ([% WHILE ( part_numbers.$i.defined OR part_names.$i.defined ) %])

I don't think it should expect a filter on that line.