Mac OS X: Drawing into an offscreen NSGraphicsContext using CGContextRef C functions has no effect. Why?

Mac OS X 10.7.4

I am drawing into an offscreen graphics context created via +[NSGraphicsContext graphicsContextWithBitmapImageRep:].

When I draw into this graphics context using the NSBezierPath class, everything works as expected.

However, when I draw into this graphics context using the CGContextRef C functions, I see no results of my drawing. Nothing works.

For reasons I won't get into, I really need to draw using the CGContextRef functions (rather than the Cocoa NSBezierPath class).

My code sample is listed below. I am attempting to draw a simple "X". One stroke using NSBezierPath, one stroke using CGContextRef C functions. The first stroke works, the second does not. What am I doing wrong?

NSRect imgRect = NSMakeRect(0.0, 0.0, 100.0, 100.0);
NSSize imgSize = imgRect.size;

NSBitmapImageRep *offscreenRep = [[[NSBitmapImageRep alloc]
   initWithBitmapDataPlanes:NULL
   pixelsWide:imgSize.width
   pixelsHigh:imgSize.height
   bitsPerSample:8
   samplesPerPixel:4
   hasAlpha:YES
   isPlanar:NO
   colorSpaceName:NSDeviceRGBColorSpace
   bitmapFormat:NSAlphaFirstBitmapFormat
   bytesPerRow:0
   bitsPerPixel:0] autorelease];

// set offscreen context
NSGraphicsContext *g = [NSGraphicsContext graphicsContextWithBitmapImageRep:offscreenRep];
[NSGraphicsContext setCurrentContext:g];

NSImage *img = [[[NSImage alloc] initWithSize:imgSize] autorelease];

CGContextRef ctx = [g graphicsPort];

// lock and draw
[img lockFocus];

// draw first stroke with Cocoa. this works!
NSPoint p1 = NSMakePoint(NSMaxX(imgRect), NSMinY(imgRect));
NSPoint p2 = NSMakePoint(NSMinX(imgRect), NSMaxY(imgRect));
[NSBezierPath strokeLineFromPoint:p1 toPoint:p2];

// draw second stroke with Core Graphics. This doesn't work!
CGContextBeginPath(ctx);
CGContextMoveToPoint(ctx, 0.0, 0.0);
CGContextAddLineToPoint(ctx, imgS