C# StringMap (and probably more) broken

[Simn]

My recent change somehow broke StringMap on the C# target:

function main() {
	var smap = ["foo" => 1];
	var index = smap["bar"];
	$type(index); // Null<Int>
	trace(index); // 0
}

I have no idea why as of yet. There's a comment in gencs.ml which sounds like it's related, but I'm having some trouble parsing what this actually means.