toLowerCase() fails in assertion

I'm trying to embed some trivial script in my assertions, and running into this strange problem: toUpperCase() works as expected, but toLowerCase() has no effect. Specifically, I'm using a CONTAINS assertion containing context references, as in this example: ${=context.firstName.toUpperCase()} upcases the property value as expected for the assertion. When I call a similar function like ${=context.contactEmail.toLowerCase()}, however, it returns the original mixed-case string.

I've tested these calls in a script step and they both work as expected, it's only a problem when the calls are embedded in an assertion (where there's no specific import, etc.) in case that sheds some light on the problem. I guess I'll create a script/property to work around this problem, but hopefully we can discover a solution so I won't have to rely on the workaround.

Thanks in advance for any help you can provide. -jn-