Emacs indentation of objective-c block syntax

Question

I'm using objc-mode and in general the indentation works quite well, but I am having difficulty with the following code.

dispatch_async(dispatch_get_main_queue(), ^{
    [self someMethod];
});

Emacs tries to indent it like so:

dispatch_async(dispatch_get_main_queue(), ^{
        [self someMethod];
    });

Basically whenever there's a block inside a function / method call, it indents too much. The syntactic analysis for the 1st line of the block is (arglist-cont-nonempty, statement-block-intro). statement-block-intro is set to '+. arglist-cont-nonempty is set to '(c-lineup-gcc-asm-reg c-lineup-arglist). In other words, just the defaults.

What should I do to get it to indent properly?