I think the problem is more general than simply a MarkupParser problem. But in my case, the problem occurs with MarkupParser.

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public static int main (string[] args) {
    MarkupParser parser = { null, null, null, null, null };
    return 0;
}

$ valac test.vala
test.vala:2.18-2.58: warning: local variable `parser' declared but never used
    MarkupParser parser = { null, null, null, null, null };
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
/home/seb/programmation/vala/test.vala.c: In function ‘_vala_main’:
/home/seb/programmation/vala/test.vala.c:21: error: ‘GMarkupParser’ has no member named ‘start_element_target’
/home/seb/programmation/vala/test.vala.c:23: error: ‘GMarkupParser’ has no member named ‘end_element_target’
/home/seb/programmation/vala/test.vala.c:25: error: ‘GMarkupParser’ has no member named ‘text_target’
/home/seb/programmation/vala/test.vala.c:27: error: ‘GMarkupParser’ has no member named ‘passthrough_target’
/home/seb/programmation/vala/test.vala.c:29: error: ‘GMarkupParser’ has no member named ‘error_target’
error: cc exited with status 256
Compilation failed: 1 error(s), 1 warning(s)

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The C code generated:

GMarkupParser _tmp0_ = {0};
GMarkupParser parser;
_tmp0_.start_element = NULL;
_tmp0_.start_element_target = NULL;
_tmp0_.end_element = NULL;
_tmp0_.end_element_target = NULL;
_tmp0_.text = NULL;
_tmp0_.text_target = NULL;
_tmp0_.passthrough = NULL;
_tmp0_.passthrough_target = NULL;
_tmp0_.error = NULL;
_tmp0_.error_target = NULL;
parser = _tmp0_;

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It's a regression in Vala 0.13, since with Vala 0.12 there is no problem.