Why is this illegal in strict mode?

Yeah, yeah, I know, strict mode ain't around yet, but really, I'm planning for the future...

So, why is this:

$('#'+ $(this).attr('id').replace('control-', 'legend-')).fadeIn();

... not allowed in ES5 Strict mode?

Or am I misinterpreting? JSLint:

Problem at line 516 character 18: Strict violation.

Could it be a little more verbose, I wonder...?

EDIT:

To avoid confusion, here's more of the original code:

function displayLegend() {
    $('#'+ $(this).attr('id').replace('control-', 'legend-')).fadeIn();
}