Single quotes in mathtext

Jos?> Hello, I think I've found a bug in mathtext - sorry if this
    Jos?> appeared before, I couldn't find it in the list archives.

    Jos?> When I try:

    Jos?> xlabel(r"$-(\Omega_a^' + \Omega_a)$")

    Jos?> I don't get what I expected. The label shows -\Omega_a and
    Jos?> stops there. Further tests showed that, whenever I try a
    Jos?> single quote in a mathtext string, the text is not rendered
    Jos?> from the quote on.

    Jos?> This might not be a bug, I might be doing something
    Jos?> wrong. If this is the case, please tell me how to do it
    Jos?> right.

I don't believe your use of ' is valid TeX. Try
Omega_a^\prime

    Jos?> Thanks in advance.

Sure thing,
JDH