Either update to a version of S2 that includes the bug fix or find the bug fix and patch whatever version of Struts you're using.
Searching further, I found this:
https://issues.apache.org/struts/browse/WW-2176
It doesn't look like it's been fixed, although I think the bug is actually in XWork rather than Struts.
If that's the case, another workaround would be to use a custom type converter rather than relying on the XWork one (again, if it isn't really fixed--have you tried the same thing in a recent version? For some reason you won't tell me what version you're using so I can't really try a more recent version, since I don't know what a "more recent version" would be).