initWithFrame not called, but awakeFromNib is

I am trying to subclass NSOutlineView. Here is my code:

OutlineViewSublcass.h:

#import <Cocoa/Cocoa.h>

@interface OutlineViewSubclass : NSOutlineView {

}

@end

OutlineViewSubclass.m:

#import "OutlineViewSubclass.h"

@implementation OutlineViewSubclass

- (id)initWithFrame:(NSRect)frame
{
    self = [super initWithFrame:frame];
    printf("debug A\n");
    return self;
}

- (void)awakeFromNib
{
    printf("debug B\n");
}

@end

The debug output is:

debug B

Why isn't (id)initWithFrame:(NSRect)frame being called?